For anyone who hasn't figured it out yet, the idea of the Tower of Trials is that, against the earlier opponents that are weaker than you, you accumulate and save SP. Then against later opponents who are comparable to or stronger than you, you can spam attack skills that make you invulnerable while the skill executes. Use the skill however many times it takes to finish off your opponent and then you win. Be careful not to let any characters die against earlier opponents, as that means that character's accumulated SP is gone.