The absorption mechanic is really cool, I also thought the spear move felt really satisfying. The death upgrade thing is a neat idea, but I think it needs more work to integrate with the absorption mechanic because on my first play through I focused on dodging the attacks, took almost no damage and only really unlocked the first upgrade right before the end.
It seems like the best strategy for speedrunning is to just take as much damage as possible until you unlock infinite ammo, then just rain destruction down from a safe place.
My suggestion is to either limit the jumping so you can't get above the enemies (because they can only aim at the ground) or make the enemies more dangerous and have them aim at the slime wherever he is rather than just on the ground. I also think it might be cool to replace the infinite jump with a double jump, but make the slime stick to walls somehow (he is a slime after all).