I do not think highly of this game, partly because a good third of the achievements here are invented by some lunatic or even Alpha Centauri inhabitant. And while there are dozens of guides on the secret doors or better set for team skills, until this moment, you can't find a thorough guide on all 30 achievements.

So there we go. Before we start: to make a full completion of this game, you’ll need to finish it three times minimum. And do some other boring stuff. Just a warn off.

If you have questions about the best ways to get achievements, feel free to ask.

These five badges you'll get any way you make through the game:

Exterminator - defeat Spider Queen in the first major quest

Hired - Complete 1 act

Rebel Yell - Complete 2 act

Into the Abyss - Complete 3 act

Lepidopterist - Complete 4 act. By the way, that word means the man who studies butterflies (vivisect them, too)

Now, the most challenging: three achievements supposing that you need to complete the game 3 times, and every time in the misbalanced crew:
All might all night - complete game with 4 fighter classes (Mercenary, Blade dancer, Defender, Barbarian)
Hybrid heaven - complete game with 4 mix-classes (Crusader, Ranger, Scout, Hunter)
Magic Menagerie - complete game with 4 magi-classes (Free mage, Druid, Rune Priest, Shaman)

Might & Magic X tends to vary enemies during the major quest, so there will surely be some fights very tough for mages and many situations where fighters will be in a very uncomfortable state without some helpful magic. So every one of those imbalanced teams is surely a hard time.
The easiest of those 3 achievements considering completing the game with certain classes, obviously, the middle one. While crusaders and rangers cannot be the best mages, they, at least, have powers to keep all alive. In the end, that's all that matters.

But, I'll not give you a 100% receipt for the best teams there. As for me, I've made all three noted achievements with max variety possible - every time I complete the game with the human, elf, dwarf, and orc, all tuned up to higher classes. It is the longest way, but the only one possible for getting a least some variety of that pseudo-old school sub-RPG. And the game not that long anyway, only uncomfortable compared to the modern games like Skyrim or even in the same class - to Legend of Grimrock.

Also in that section,
Unbreakable – complete the fourth act without the need to revive anybody from your crew. Possible, if you have a mixed powers team with good healers. Just power up your team before going into the end-game dungeons.

Keep on Running – complete fourth act w/o using spirit beacons. Of all transport measures, the spirit beacons are too pricy for everybody besides the team of mages. And even then, it is the most limited and uncomfortable way of fast travel. You can live easily without it.

Paragon – get all upper classes for your team. This is a sub-quest-related achievement (you need to finish 4 separate sub-quests, for every teammate), but I put it in this section for its usefulness. You can complete the game without finishing any sub-quest… But in that case, you’ll not see the good half of the game, and you’ll be operating the team of weaklings. Only upper classes can be the grandmasters in the given skill, see? So get them all.

They could be missed, but I suggest any adventurer get all of them. If you can't, I suppose, this game does not suit you.

Corners of Earth , a short quiz for the geography of Eryn, from lord Kilburn. You'll find him very early in the second chapter, during the main quest. Pass the quiz - take an achievement and the strongest shield in the game.

And yet it’s flat! - there is an astronomer guy, Edwin from Seahaven, who needs to be escorted to some corner of this little land. While he is in the group, you take a +10% exp bonus - for free! Another character with the same trait asks 10% of gold for his service, but Edwin just hopes that you'll finally bring him to the place he wants to see. That makes him one of the most valuable companions in the game, since the exp points are scarce, really. I'd recommend getting Edwin as fast as possible (very early in the 2nd chapter) and hold him as long as you can. Until you'll strengthen up to 20+ levels, at least.

Umberto's Echos - one of the longest quests in the game, and one of the least useful unless you really need that badge and all the exp points you can get. In short, you can finish it only after you finish the main quest. You need to speak to Umberto, a sage from Seahaven, and then find 9 books for him, scattered across all Eryn. After you collect them all (and kill the bad guy in the process), you return to Umberto to gain the badge we’re talking about, some exp and unique powerful item, totally worthless, considering the fact the bad guy is dead.

Haart but fair – just complete the quest about Haart fella, who has a residence in the Seahaven and personal spy – inquisitor nearby (a quest giver, really). The quest has two solutions, moral choice involved, but the one where you speaking the truth creates you more problem and nets a bit more exp. Karma and EXP points… The choice is yours.

Catch them all – the game automatically writes and rewrites hints about enemies you’ve met. You just need to come to Jassad fella in the tavern of Sorpigal, to share the data. After you make 50 notes, you’ll get a unique dagger from him. Worth the trouble.

Behind the scenes – Aftermath quest. You’ll get a key for that dungeon only after completing the main quest, from the fortune-telling gypsie. All dungeon is a total mess, but it is rather easy and fast to deal with. I doubt you’ll wanna return there, though.

Most of them will be of missable, but manageable type. Like that one:

Slayer - kill 1000 enemies. I guess, the number is counted around all your efforts. So I'd expect that to pop up on the second or third run through the game.

24/7 - that's an easy one, but very boring. Your team cannot die of insomnia, they just lose their power to fight properly. So you just don't go to rest for one in-game week, walking around, back and forth. It could be performed even before you clear the spider lair very early in the game, but I suggest you leave that fishy badge to the times when your team will be of 20+ levels, and most of the map will be revealed. You may complete it while doing one of the next two achievements on my list.

Cartographer - explore 100% of Agyn peninsula in one walkthrough. Well, that doesn't mean you should step on Every Square on the map. Just cover more than 70-80% of tiles and discover all regions on the Agyn. Considering the size of the map, it is not so hard, but obviously kinda frustrating - to waltz back and forth over and over, enlightenin' those "dark" tiles on the map.

Obelisks - now, that one is neat, comparing to all other side quests and achievements. First, the obelisks are free teleporters between different regions in the peninsula. The downside is that you should find them to start, and activate, to use them as a transport system. But again, they are free, and often much more convenient than other quick travel matters in the game.
After you find all 8 obelisks, you need to gather all their mumblings together, return to the one near Portmeyron castle, and collect a powerful 1h sword, possibly the best in the game (but totally useless for mages, of course).

Priceless - very easy achievement, you need only to find 5 gold (unique) items to fit on one person in your team. It can be done even in the first chapter if you spend 30 Ubi points to unlock the "special relics set" in the rewards section of the game in the Ubi browser. Those are very useful 4 items, the best of four "rewards" Ubi suggesting. Well, if you are not playing for one of those 3 teams from section 1, and gather your own, balanced and rightful gang of true adventurers, maybe you don't need to unlock that reward. In the end, you'll find more than enough relics on the way.

Dr Jones - collect 20 relics. In fact, there are more than 20, so you'll manage. Again, it will be slightly faster with the "special relics set", but can be done without that set. If you are the true adventurer not avoiding all the dangerous caves and mysterious crypts, of course.

Scrooge - collect 50000 gold. Easy one. You just need to find more magic items and sell all of them. There are two helpers for that cause - one is called Huang (can be met on the way to Crag near the broken cart in Desolate Wilds), he's a trader who will buy everything from you. Second is Zhin, an adventurer from the Crag, she'll make all items drop more magical-likely. And more magic means more shiny things, even if those two "helpers" will take their share of it. Closer to the end of the game, you'll be founding chests with 3-4 thousand coins each and items with sold prices over 1K each. So 50000 is an easy mark. Unless you train the party of mages...

Walking Library - learn 35 spells with one party member. For one, they really need much more supplies! And secondly, all those spells are quite pricy. The easiest way to collect an achievement is to create a jack-of-all-trades free mage and buy the cheapest spells for every magic school.

Fortress "the party blocked 5 attacks in a row", says the description, but I found it not completely correct. See, I've created a team of 4 defenders, taught them to use shields, found a single non-magic enemy, and start counting. Well, I'm certain I gain 5 blocks-in-a-row several times, but the game doesn't count it. Next, I create a magi team, to complete Magic Menagerie. There was only one shield user (I needed at least one decent fella to stand the ground, and of course, it was dwarf). I made him master of the shield and gave him a unique shield from lord Kilburn (it gives +1 block attempt). And that's how I finally got that achievement. You see, if you have some fighters in your team, they will be dodging hits. And dodges not counting as blocks. So you need a team (or maybe just one guy, for that case), who is not good with dodges, but very good with the shield.

Walking Dead – revive your teammates 100!!! times. Depending on your skill and style of playing, the difficulty you choose will be either one of the easiest collectibles from all 30 or one of the worst. I can’t stand the thought of killing my teammates on purpose, so I've not achieved that badge even after 5 times I’ve been through the game...

No seasons – spend 336 days in one playthrough. Usually, it takes about 100-150 days to finish all the stuff in the game, with all quests, all enemies, even all the tiles covered. To get that badge, I suggest saving about 100K of money (maybe less), and constantly pay for the rest in the inn. One in the Karthal suits fine – you just click the “1” button for about 20-30 minutes, and the badge is yours!

Sonic Hour - walked 123500 tiles. Remember the Cartographer badge? Well, it is nothing, comparing to that one. I'll be honest with you, I still not get it myself. Totally useless and mindless achievement, I think it will pop up after 7-10 games through... if you manage that much.

To the Max – reach level 40 with all 4 heroes, in one walkthrough. Well, hardly a believable achievement, but there is a possibility. I’ve already mentioned one researcher who gives a +10% exp bonus, but there are two of them! So, after you finish the first chapter, killing as few mobs as possible, you go first to the Seahaven, meet Edwin. Then make the fastest way possible to the Karthal Slums, where you find Haji, another scholar, in the local library. And only after you've got them both, you'll start hunting mobs and solving quests. Sadly, but this kills all the fun out of the game. Without scholars bonuses, the end-game party will be at 30-35 levels, with all killed and done.

But, what else I can suggest to you? As I said, this achievement is very unlikeable - and game devs already get their paychecks, hardly they look back to fix something about M&M X anyway. So - tweak the statistic, cheat it – as I said before, nobody will care! This game is left behind, there will be no new addons for a fair possibility to meet the level cap. The only advice is to not doing cheats before you finish the game fairly – you come to play for a challenge, for the first time, remember?

Anyway, I wish you good luck on your adventure!