The girl who just spoke is clearly being a pain in the ass, and after she leaves, one of the boys says this :
1 - オレ憎たらしさには 自信があったが あいつだけはぜったい勝てん。
I was sure I was unpleasant, but it's nothing compared to her.
I'm used to see 勝てる used with に, which here is replaced by だけ which confuses me a bit.
It doesn't seem to mean "only her", or maybe in the sense "I'm still more unpleasant than anyone, except her"?
The thing is the contrastive は combined with に seems to have this meaning already, "I can't win against him VS I can't win against him (though I would win against others)"
in this question : sometimes だけ gets mildly confusing.. someone suggests that in example 2, だけ is juste used to stress は.
2 - それだけは、食べないでください。 "Please don't eat that. (Eat anything else.)"
Long story short, what would be the difference in example 1 if だけは was replaced by the following (my guesses on the right)
に - Just a simple relation : I'm nothing compared to her.
は - Same as には but with a casual drop of the に as in 僕（に）は自信がある
には - Contrast : I'm nothing compared to her (though compared to ohers...)
だけ - Stressed version of に with a casual drop of に : I'm nothing compared to her (and her only, don't know about the others)
だけは - Stressed version of には with a casual drop of に : I'm nothing compared to her (and her only, though compared to others...)
だけに - Plain version of だけ
だけには - Plain version of だけは
All the versions without は dont feel right to me in a negative sentence.