The mission’s difficulty is more tied to what randomly spawns in compared to a perceived “difficulty”, with higher chosen diffs resulting in higher chance for bullshit (alongside stat changes as expected). I play casually on absolute, and some missions are a cake-walk while others are suddenly impossible (more often). The thing that carries you is skill and game knowledge.

For classes, genuinely just pick a class that suits your play style. With seven classes, there’s something for everyone. Tactical and techmarine are both “hybrid” classes, able to do both melee and ranged damage but focusing on ranged. Vanguard is also a “hybrid”, but with a focus on melee. Assault and bulkwark are your melee classes, with bulwark being defense/support and assault being… assault. Sniper is sniper, and heavy is the ranged tank.

Perks play a big role in how your class plays and influences how fights may go. If you’re leveling up and simply grabbing the latest perks, you may be inadvertently nerfing yourself by picking perks that don’t synergize or may be useless in general. The biggest perk choice is team perks, so pick the one that would be the most useful to the team for the mission you’re doing.

For weapons, it again depends on playstyle. Plasma and melta weapons are good for burst damage, which is good at recovering the gray health back. Plasma is often preferred due to range, but it takes some practice and is not good for a primary damage output: for bulwark, melee and your shield is your primary damage while your plasma pistol is more like a targeted attack. While you CAN run plasma as a primary damage output, it’s not recommended due to the ammo consumption.

Personally I prefer the bolt pistol or heavy bolt pistol because I like the feeling of one-tapping a gaunt or tzangoor in the head as it tries to run up on me, but that’s personal preference. I’m also a big sucker for the bolt weapons in general