I have taken all of them to OP8 solo, well sort of, some with splitscreen help (I got 2 accounts)
I run Salvador and Maya to OP8 solo, legitimately.
Then I run Krieg, Axton and Zero in UVHM until I got to Bar Room Brawl and powerlevel myself to 72 (with my OP8 Salvador). Farmed for 72 gears then run the peak up to OP4 solo.
Then I run 2 more peak runs with OP8 Salvador to get them to OP8. Why only 2 peak runs? Because of the Peak free OP levels skip (or whatever they call it). You need to complete the final area in the peak and proceed to the elevator, once the mission lights up green, go back to the last area, once there, I use the one i’m powerleveling to turn the mission in and start a new peak mission. after that just walk in the entrance of the last area and you can clear the map and complete the peak again to unlock another OP level.
For Gaige, I haven’t run a complete playthrough yet with her, I just powerlevel one to do some raids.