Bent shift lever on Shimano Deore MTB shifter
It’s the bottom lever that is bent right?

You are just going to have to find some way to apply leverage to the thumb lever, I.e., bigger wrench or pliers.

An approximately 30cm length of metal or stout plastic tube that just slips over the shift lever is ideal. 

Be aware that the aluminum lever is going to be weaker at the spot it was bent and bent back.