There is a threshold where overcoming a challenge is no longer rewarding, and Space Marine 2 hits that mark in high difficulty operations early on. Enemy mobs feel way too tanky and unbalanced. Like, there is some enemy designs that feel deliberately frustrating in the name of difficulty (zoanthropes). Somewhere on the way I think the devs forgot to ask if it was still fun to play, not just that 'it works'. There are some impossible scenarios, where if it were in Helldivers it would be super fun and chaotic, but in SM2 if you stop parrying the 10 hormagaunts attacking yo at the same time you'll get stun locked for the rest of the game. 

I've tried Lethal many times, but between the matchmaking/connection issues, pointlessly stupid companion AI, the length of missions and an AI director that doesn't know how to balance mobs of unbalanced enemies, I've just given up on it. I hope that the difficulty gets reigned in a bit, because I'd like to play these modes and say I had fun. But as it is now, even substantial has me rolling my eyes, it's not that hard in general, but the encounters just get monotonous which is magnified on higher difficulties. I'll forget the special helmet, even if it does suck to be barred because of a what I think is a skill gap coupled with lazy design. The game feels best between average and substantial difficulty, were that tweaked to have larger mob density and more enemy variety, it would make for a more interesting challenge on higher difficulties. But just stacking enemy health and damage to absurd levels? Nah, Ima pass. Lower difficulties are still fun, but it sucks that the rest of the games variety is a frustrating experience. I'm sure many masochists have "thoughts".

It'll be tough to compete solo with bots ruthless and above, not impossible, (i really enjoy solo ruthless when i cba playing with other people), so the only options really are to either play more and get better, or play with decent people online and get better that way. 
thats kinda it with this game - you gotta dive into the deep end, learn the mechanics and just get better at the higher difficulties, and one day you'll find that its just easier for you.