after a while of testing, i've come up with a strategy that breaks the game. i'm calling it the "solo method". here's the gist of it:
this strategy centers around a recent buff to the "resistance" cards. there's one of these for each type of enemy, and they prevent that type of enemy from targeting the doll with it equipped. before the buff, this only had any effect when the player wasn't controlling that doll, but now it's always in effect.
the other mechanic this is working off of is that, when a doll is destroyed, she drops her upgrade card on the spot, where it can be picked up by the next doll that walks over it. that doll gets the effects from both cards, including the stat buffs. you might already be seeing where this is going, but let me walk you through it.
step one: set your favorite doll and weapon to slot 1. this should be whatever combination you're most comfortable with - personally, i run the kitty in this slot since the dash ability is going to help later. it doesn't matter in the slightest what dolls or weapons are in the other 3 slots, so don't worry about it.
step two: equip 3 cards with the effects of byte resistance, kilobyte resistance, and megabyte resistance. these might be a little hard to get, but this strategy hinges around them, so don't try this unless you have all three. if you've got multiple, i'd prioritize the ones with the highest speed buff and the lowest defense buff. the last card is dealer's choice - i recommend bit magnet, soft landing, colorful floor, or just whichever one has the best stats.
it doesn't really matter that much which dolls you equip which to.
step three: as soon as you start the battle, hold E to make all ai-controlled dolls stand still. this will make it easier to find their cards when they drop them. from here, just keep playing as normal, handling all the firewalls and ball delivery yourself. try to kill as few small enemies as possible - ideally, you have exactly one clingy megabyte going on a rampage with no other megabytes on the field.
once the other three dolls go down, grab their cards and take out the last megabytes. at this point, none of the small bytes will target you at all, since you have all three of the resistance cards in effect. no more enemies will spawn, either, since there's an entity limit. this step can go a lot faster if you get the byte rush or megabyte hazards.
step four: at this point, you're essentially invisible to the enemy and can walk freely from the firewall to the goal until you cap out your difficulty. better hope you've got a good mashing finger, since you're not going to be getting any more bits from enemy kills. the only dangers at this point are the rain bombs or running out of time (that's why i recommend high speed and the dash).
i've been able to consistently hit max difficulty this way, and it's a great way to farm bits and doll exp if you don't want to use the registry. honestly, i feel like this might get patched out in some way, since once you have everything set up, the rest of the battle is a cakewalk.