TLDR for achievements only: 'cheevo's are easy and straightforward. None are missable and I followed no guides in my playthrough and got all but 2 without even looking at the list. Those were easily mopped up afterwards.
Note: This game has a couple somewhat challenging puzzles and battling is tougher than in other monster training games. It teaches you the mechanics well but doesn't hold your hand when it comes to crafting well balanced teams. Plenty of guides on Steam can help you through both of these things if you're stuck.
When I was 10 I bought my first Pokemon game. It was Blue (my mom wouldn't let me get Red version because she thought it looked like the devil was on the box....poor Charizard). It was fantastic! I couldn't stop collecting. I NEEDED all 151.
But then I grew up. I've since shifted away from Pokemon because the main game is too easy and takes little skill and the endgame (i.e. online battles) involved catching the exact right 'mon, spending loads of time EV training, etc etc. It could take ages to get a full perfect team together....and then you find out the team isn't as good as you thought so you have to go through the whole process all over again for different 'mons.
Enter Monster Sanctuary. This beautifully crafted metroidvania collect-'em-all game is everything I wish Pokemon was. One 'mon is always following you around (like Pikachu in the Poke show) but each of them has an ability. Punch through walls? check. Create rocks to trip switches? check. Mount a 'mon to ride quicker than running? check. And many more. Switching on the fly is super fast.
That quick switching brings me to my next point. The UI in this game is excellent. Each monster has a large unique skill tree so you can amass 7+ skills on some of them. Swapping those skill orders is intuitive and quick. Changing up your team of 6 is really easy. You can carry as many monsters with you as you would like and can swap them into your battle team of 6 in a flash.
All battles use 3 'mons at the same time. Buffs and debuffs are extremely important in this game. Unlike Pokemon where you could just use Charizard through the whole game and never switch, that would be extremely hard in this game. M-Sanctuary encourages you to change up your team to exploit enemy's weaknesses.
When you defeat wild monsters you get a rating on how well you did. The better you fought, the higher chance of getting egg drops of the 'mons you defeated.
DON'T hatch the eggs until you're ready to start using that specific 'mon. This is because when you hatch an egg, that 'mon is automatically set to a level that's 2-3 levels below you average current team level.
The variation between fighting wild 'mons, keeper battles (like Poke trainers), and Champion monsters, along with the Infinity Arena, and PvP, this game gives you plenty of ways to keep you entertained.
I hope if you give Monster Sanctuary a shot that you enjoy it as much as I did. I definitely plan to dive back in soon. Really hoping there's DLC on the way!