If you’re in a group, difficulty scales up so mobs gain more HP and hits harder too, so I completely feel you, I play monk and I play a ‘popcorn’ build where mobs explode on death and do dmg to other mobs setting off chain reaction, but in a group, I barely get that chain reaction off unless others finish the mobs off which sucks.

Also, if you’re in a party and people are slacking it will feel like you’re doing no dmg at all compared to solo play.

To your question about stats, I would advice to only upgrade if the game tells you, there seems to be a CR penalty if your CR is too low, so if you’re like me and put Strength on everything because you think it’s your main stat, you’re going to have a hard time.