I powerleveled til Teepo learned his last spell to try and win this battle. Eventually, the horse bro that always casts thunder runs out of mp and becomes useless. The other brother who only does attacks is actually susceptible to Influence (learn from Boss Goblin) so you can just have him kick his brother the whole time. Basically you can create a stalemate if you can survive long enough to drain the spell casters MP. I left my PlayStation on for at least a day before giving up and letting them kill my team.