For the most part, just play the game. However, simply following the 'Odyssey' quest line will leave you well below the level required to advance.

So i'll start by saying, people who know how to play the game will read this and say "well da doy son". This guide is intended for all those who say grinding is impossible and both XP and materials require IAP.

Not so.

Do side quest's, check bulletin boards or any ? on the map, YES, i know they are mundane, but crucial in advancing.

Next, DON'T ride your horse and DON'T follow roads to your next quest destination. It saves you seconds at best. Instead travel on foot, on a path as straight as the crow fly's. Take this opportunity to collect wood, metal and skins.
I can almost guarantee, if you travel on foot, you'll collect at least 20 wood, 15 metal and up to 20 skins before you reach your destination (you'll see a dot above things you can harvest even from a few meters away). And let's face it, any distance around 700m will take about 1 min.

In the same way that i started out, DON'T think you can ally with the Spartans. There is no allegiance in the game and you can't change history (you're in the animus not a time machine). Instead, go into a region, see who controls it and start a war with THAT faction (burn faction loot, loot faction boxes or just outright kill their troops) which will lead you to kill their leader, if you've done enough of the previously mentioned. You will get heaps of XP for winning a war as well as decent loot, though you will get better rewards on the attack. Remember that if you've angered a certain faction it's just a matter of paying the bounty (pffft who pays fines) or killing the person offering the bounty, to get back in the good books with the specific faction.

Your quest line will lead you to different regions, don't stop there, explore neighboring regions, raid outposts, complete timed quest's, Clear ALL region/outpost requirements, START WAR ( Always start war ). You'll find your XP growing pretty quickly.

Just because a mercenary is a few levels above you doesn't mean you should run, your assassin abilities (especially critical assassination) make it possible to kill someone a few levels above you (if the mercenary has a skull, leave them alone, and generally, they leave you alone). ALWAYS use your dodge or parry, if you cant' get hit, you cant take damage, run and hide in a decent group of cover and you can stay hidden long enough to unleash another assassin attack, rinse and repeat.

Upgrade your gear if you're struggling, personally i only upgraded equipment every 3 levels, anytime you upgrade it will match your current level (obviously costing more materials opposed to leveling each level). You'll eventually find a set of legendary armor which fits your play style, so hunt for it and keep it upgraded. Also find a balance between dismantling weapons/armor
and selling them. Weapons generally dismantle into precious gems, wood and metal, whereas armor will generally dismantle into leather, wood and metal. For the most part i sell whites and blues and dismantle purples, though as i said, find the balance to ensure you have good material stocks and good amounts of drachmae.

There are naval quest's, look at the mast of you're bireme (Now that i think about it, bireme is your boat, the main pole in the middle is your mast and it looks like a tablet stuck to the mast), which also award decent XP and specific loot.

Lastly, NEVER FORGET, this IS an Assassins creed title, as much it literally is LOTR: Shadows of war, with an AC skin, Assassinations can chain to offer bonus XP. I KNOW how much fun it is to pump everything into hunter skill's an head shot everyone from a distance so far they can't see you, but using stealth, watching enemy movement patterns and executing them as quickly as possible with assassin skills will reap XP rewards.

For all the heroes that want to laugh at this guide, I don't care, anyone who thinks i need to add to this guide for the intended audience, just say so and i will add it.

Happy assassination to you all.