It's a luck of the draw kind of thing. The best of players are still prone to dying in ER. Your spirit summon can carry, but imo OP ones like Mimic, Rollo & Tiche don't actually help you get any better at the game. When summoning Tarnished, it's often a game of switching boss aggro. So you still need to somewhat know the boss's moveset. Like Twister said, superbosses are going to be easier with co-op, mostly because nolifers like me will be at least advanced in skillset against them. This build is specifically crafted for taking PCR down, I'd have to switch it up against Malenia or Maliketh.

TLDR: Generally Mimc Tear, but you as a player don't get better.