Since pictures paint a 1000 words:
Basil deals enough DPS to take out the entire enemy wave, but because the remaining one is spamming his invulnerability buff, he cannot be harmed.
To me, this is an unsolvable conundrum, how do you defeat an enemy who cannot be harmed? 
Randall barely makes it through this wave, but he’s so weakened afterwards he lacks the strength to defeat the final wave and he’s alone.
Normally you defeat Invincible enemies by disabling them for the Invincibility duration but that isn’t helpful here.
I know you’re not gonna want to hear this but you’re gonna have to upgrade Randall and get his Yzma disk. With the Yzma disk he’ll be able to remove the invincibility from Hades which should hopefully keep Basil alive.
Before you complain about your resources I’ll just remind you that you’re choosing to do these higher level/rank campaigns with low level/rank heroes. If you aren’t investing in Basil then this campaign is basically a waste of time so either use resources on Randall or give up on the campaign as complaining won’t get you anywhere.
I already used 4000 stamina this week on some minor orange upgrades, so am loathe to spend more at this time.
And no, I don’t use Randall, so anything spent on him will be wasteful. He’s lucky I gave him Purple4.
Basil is the one I am investing in.
This campaign have red enemies, and so your allies must be red as well.
Power-ups won’t do all the job for you.
And there isn’t much to worry, this disk is one of the worst in game.
