In general, here are a couple gameplay tips on how to generally improve your score. I haven't gotten all achievements myself, yet, so please leave comments to help me improve this guide for more players!

1. Longer words are usually better in order to trigger those +5, +10, +15, etc. bonuses. Of course, this is easier said than done, so if you're having trouble with this, I'd suggest just starting with a goal of always submitting words that are at least 5 tiles long, every time
   
2. It's also helpful to keep an eye out for common prefixes & suffixes that can help elongate your word. For example, if you've found the word "jest", look to see if you can make the words "jester", "jesters", or "jesting". Common prefixes include "re-" and "un-". Common suffixes include "-s", "-ed", "-er", "-es", "-ful", "-ier", and (my personal favorite) "-ing"
   
3. Don't be afraid of using the refreshes! As the game goes on, you'll likely see more and more vowels remaining on the board, or sometimes you'll just get a bad shuffle, and that'll make it hard to make lengthy words. There are also modifiers that can help you keep your tiles fresh, such as refreshing after every round, or an upgrade that lets you shuffle specific letters
   
4. Perk synergies will help, so try to build towards those, though that is luck-based. As an example, I once got a modifier that turned a random tile golden every time I refreshed. So, I made sure to get modifiers and upgrades focused around refreshing, so I could stockpile gold tiles in my word bag and make it easier to get multipliers
   
5. You can manually rearrange tiles on the board to help you see words. I like to keep my commons prefixes & suffixes grouped together, for instance, so it makes it easier to spot when I have an "-ing" I can tack onto the end of a word
   
6. If you are having a hard time, there are word unscrambling tools online that you can use as an assistive last resort. Someone has also written a Steam Guide built specifically for Word Play's dictionary: https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3527537715

There are two achievements that you'll get over time as you play more Word Play. You can check your stats on these in the "Extras" portion of the menu.

1. Eloquent - Submit over 1000 words
   
2. Can't Stop Winning - Win 50 games in Strategic mode. Quickplay doesn't count, and otherwise the fastest way to get this is by repeating a bunch of "easy" mode games as they're the fastest to complete

There are 7 total achievements that trigger when you successfully complete all rounds of a selected game mode for the first time.

1. No Problem - Complete Easy Mode
   
2. Average Gamer - Complete Normal Mode
   
3. Tough as Nails - Complete Hard Mode
   
4. Total Wordsmith - Complete Legendary Mode
   
5. Unstoppable - Complete Marathon Mode
   
6. Podium Finish - Complete Quickplay Mode
   
7. Word Perfect - Complete Ultramarathon Mode <-- unlocks when you complete both Legendary & Marathon Modes

With the exception of Ultramarathon Mode, you can complete these in any order. For me, I did easy, then normal, then quickplay, then marathon, then hard, then legendary, and am currently working on ultramarathon.

For quickplay, the main thing to keep in mind is that you want to maximize your score each round based on the modifier you picked, and that rounds are only 4 word submissions long. Special rounds give you more points, so choose those if you feel confident you can complete them.

For marathon, it's best to build towards multiplicative modifiers, upgraded tiles, and ways to increase the number of plays you have - there's an excellent modifier that converts all of your refreshes to plays, and potion tiles will also increase the number of plays you have. I generally start struggling at round 18, as the score goal starts getting very large.

The rest of the achievements are triggered based on what words you submit and what modifiers/upgrades/gifts you choose. In no particular order:

1. Incredible - Spell a word with at least 10 letters. This counts letters, not tiles, so gift tiles like "-ing" and "-eres" will also add even more towards this
   
2. Beyond Incredible - Spell a word with at least 20 letters. You can only ever have a maximum of 19 tiles on the board (assuming you grab modifiers that give you +3 tiles spaces on the left side), so you'll also need to get at least a couple multi-letter gift tiles in order to get to 20 (e.g., "-ing" or "eres"). You may also be able to do this with mirror tiles, or if you happen to get a special round that already provides a letter for you at the start. This is another achievement where you'll likely need a letter unscrambler
   
3. Super Scorer - Score 400 points with a single word. I found this easiest to achieve by maxing out diamond tiles to 99 points, using gold or dot tiles as multipliers, and choosing modifiers that would multiply points for tiles in the second/fourth slot or if it ended with a certain letter
   
4. Super Lucky - Have 3 Emerald tiles trigger in the same word. I got this by getting upgrades that would convert tiles to emerald, getting a modifier that would randomly turn a tile emerald if it were only 5 tiles long, and, when I felt like I got enough emerald tiles, getting a modifier that would guarantee an emerald trigger (at the cost of likely destroying the tile). There's also another modifier that increases the chance of triggering emerald tiles if there are more than one in the submission
   
5. Hoarder - Have at least 20 refreshes. You can choose refresh gifts and/or modifiers that will give you more refreshes if you fulfill conditions like starting a word with "re-"
   
6. Big Points - Score at least 200 points with a single tile. See tips in "Super Scorer" for similar strategies
   
7. Uncut - Get a Diamond Tile to at least 99 points. I believe the only way to get diamond tiles is to get a gift, or to get a modifier that upgrades your tiles in certain conditions. Once you get a diamond tile, avoid refreshes (as that'll reset the points counter) and avoid submitting the tile until it ticks up to 99. I usually put mine in the bottom left corner so I can ignore it, and so it doesn't get impacted by special round modifiers that impact the top row of your tiles
   
8. Mind Over... - Spell the word "magnet" (this also works if your word contains the word magnet). This is a reference to Mark's other game, "Mind Over Magnet"!
   
9. Midas Touch - Submit 10 Gold tiles in a single word. Similar to "Super Lucky", you'll want to choose gifts and modifiers that give you more golden tiles. Once you have enough golden tiles, if you're having trouble making a word that's at least 10 tiles (not letters) long, you can use an online word unscrambling tool (see the "General Gameplay Tips" section in this guide)
   
10. Hey, that's the game's name! - I haven't gotten this one yet, but apparently you have to spell "wordplay" as a submission, no spaces or "+" tiles necessary
    
11. Stuffed - Have at least 150 tiles in your Letter Bag. There's a modifier that gives a bonus extra tile when you add new tiles, as well as a modifier that will add a new tile into your bag if the word submitted is only 4 tiles long. You'll also want to grab any Upgrades or Gifts that give you new tiles
    
12. A to Z - Complete the "Next Letter in the Alphabet" Challenge. This is a challenge modifier that rarely shows up, and can be pretty difficult to complete, as you cannot skip a letter each time you submit a word. Additionally, make sure you don't submit multiple sequential letters in a single word, as it won't count (e.g., if you submit the word "abbey", it'll only count for the A word, and won't count for the next B word). It'll be helpful to have Upgrades that let you pull specific vowel or consonant tiles (including a modifier that treats Y as a vowel), as well as plenty of plays and refreshes. There's also an important specific modifier that lets you keep certain tiles when you refresh. I found it helpful to line up and hoard upcoming letter tiles so I knew not to accidentally submit them - rare letters like F, H, K, X, V, W, Y, and Z are especially important not to miss in the submission order
    
13. W*ldc*rd - Spell a word using only asterisk tiles. Wildcard tiles are only available as gifts, but once you get one, you can use strategies from "Stuffed" to increase your asterisk tile count. There are also upgrades that can help you get this (mirror, glass, conversion, etc.) even if you don't have a total of 4 asterisk tiles in your hand (the minimum number of tiles you must submit per word)
    
14. Mouthful - Submit three words at once, using + tiles. The "+" tile is a Legendary gift, so it's pretty hard to get it to show up - the first time I saw it was 31 hours in of playing the game, during a normal mode game that I continued into round 16 of endless. The "rare and legendary perks are more likely to appear" and "1 free perk refresh per round" modifiers helped me get the gift to even appear, then I was lucky enough to get the "make 2 more copies of a tile" upgrade the round after. Once I had at least 2 "+" tiles in the letter bag, I used up a stockpile of my refreshes to get them to show up together on the board and chained 3 words together - upgrades like glass tiles and modifiers like "add an extra tile when you add letter tiles" would also help.