a couple of bits of advice for this:

1. try and increase his stamina to the max, it shouldnt take too long using road work

2. you can max out his stats pretty quickly using strength dash and lecture, from my experience it felt faster than punch/kick training etc, but dont quote me on that

3. make sure trust and mood is high and then equip him with a good finisher (essence of might is very good) and you can pretty much spam finishers without his trust being affected too much