Spent way m ore time than I intended to on this, mainly because my first three attempts were so bafflingly difficult I could not figure out what the intention could've possibly been for this design. 300 slime kills with 3 cores across the three runs. Found your gameplay video.... oh, you just stop straight down and there aren't any there at the start? oh....
I think you should probably start the player on the ground, because starting in the air the first time, messing around with controls a bit, the bottom was overridden with slimes making killing them and more importantly collecting cores far too difficult. Knowing the strategy is to just start at the bottom and kill them before they amass too big... well, I ended up letting myself die after getting 50 orbs and killing 6000 slime.
I think all the mechanics about readjusting your aim are neat but I'm not sure how necessary they are. Probably should have a cap on number of orbs, and definitely give the player a way to magnetize cores to themselves.
Controls could also be tighter, more juice on hits and getting hit, music's a bit repetitive, sound effects stack up in that annoying way they tend to with unity... but overall I really think this was a solid game with an almost tragically poor way to start the player, at least in my case.