Enemies in the peak don’t have the same weaknesses as outside of there. Corrosive is best for most enemies. Oney is resistant to explosive, shock works well for him.
I actually enjoy the peak when it isn’t making me pull out my hair. In case you haven’t seen them yet:
These are fantastic travel guides for while you’re there. It will test you, but you will feel so badass when you triumph. Safe travels, and good luck. Let us know if you need anything.