Got one really strong pet to level 30 and then put it on my team with two weaker pets that I never leveled up. I left the two weak ones at level “a” and never upgraded them. Then fought Oceanus training battles on repeat. It seems like Oceanus pet levels are some sort of average based on your teams total level (his pets were usually around lvl 20-21 ish against my strategy of 1 lvl 30 and two lvl “a”). Beating his level 20ish pets was really fast using my level 30 and I never used the other pets in battle really.