Problem with levels

when I was battling Xavier my D. Charizard was taking out all Xavier’s team with two shots. Until he brought Herracross. My Charizard survived with 1 HP, and whilst trying to heal it, I found out that it was nearly 20 levels higher than rest of my team that was so weak that Herracross OHKOed every pokemon that I sacrificed to heal Charizard.
So, does anyone know how to quickly level up my team? It’s all level 19-21 except for D. Scyther being level 25 and D. Charizard being level 45.

Tbh ive had the same problem… Get a secret base and get the level trainer or whatever it is called…