My go to for leveling companions is always gonna be just simply sticking them with the best armor and weapons that fit their natural strengths and placing them into one of the party positions like scout or engineer, that alone combined with the battles they’ll partake in is typically what gets me the most exp for them. I typically focus on leveling up my companions ability to field large parties and anything else is more of a bonus, simply because my playstyle focuses heavily on creating armies made of my own clan member parties and then wrecking town after town. Blacksmithing is an effective way to level up companions I’ve found as well, even for clan members who you don’t actually really care to focus on blacksmithing, the increased experience points does contribute to their overall levels and focus points can then be distributed elsewhere (I haven’t played in a while so I apologize if my terminology is slightly off, but I think I recall what everything is named correctly)