Never played the merge, but:
- Pay attention mainly to whether you can expert/master/grandmaster a skill. Sometimes their bonuses make or break the entire skill (e.g. missing out on adding your skill to damage). Others are irrelevant (e.g. for armor/shield skills the most important breakpoint is eliminating recovery penalty on your attacks since DPS is king).
- Stats are weird as fuck. It's like D&D in that only specific benchmarks (for D&D even numbers) add a +1, but for M&M the amount needed goes up massively over time. You can see the numbers here:
https://grayface.github.io/mm/mechanics/. What this means is that your stats matter a lot in the early game but literally not at all by the mid and late game. A 25 might character vs a 5 might character has a +10 to damage, but by the midgame when you've hit a few shrines, drank some black potions, used some barrels, a 125 might character has only a +1 advantage over a 105 might character, and they'll be doing like 50 damage per attack so its only a 2% increase while at level 1 you're dealing like 10 damage per attack so its a 100% increase. You do need specific levels of stats to unlock certain things but generally temp stat boosts let you qualify.
- Don't be stingy on spending skill points. Like me keeping bow for everyone at 1 despite killing 90% of enemies for the first 5 hours with bows. I still do it anyway but don't be me.
- The things you want to rush are generally water and air magic because they both provide fantastic quality of life spells along with Sparks being the workhorse shotgun damage spell for your elemental casters. You want to rush to unlock light or dark magic but they are powerful enough at what they do that you don't need to stockpile points and wait for them, just a few ranks is enough for them to be overpowered (of course you do eventually want to make them your strongest skills).
- Almost all spells that inflict a status ailment on an enemy either don't work or work so rarely they are useless.
- Skills not to care about: Identify item (Get a scholar hireling to identify everything, don't worry about it early because shops are nice in M&M and pay you full price for unid-ed items), Identify monster (lmao wtf), Learning (the bonus XP is only from kills which is a fraction of total XP meaning it takes forever to pay off, though this might be different in merge if you're doing all 3 games I dunno). You can also kind of ignore Earth magic as the least useful school, though the Telekinesis spell can replace the Disarm skill.
- Assuming you're playing with Greyface patch I recommend trying shooter mode (turn on in the .ini). It's kind of an overhaul to how the game plays. Check the readme for how it works.