I’m pretty sure the most reliable approach for the lightning attacks is to jump straight up, wait a moment to make sure you absorb the lightning, and only then press the attack button. If you attack too early it doesn’t work. Maybe you already know this but it seemed reliable once I focused on the timing.
@Effidian Use the trainer to practice defense only. You don’t even need to attack him. If you can read his attacks perfectly 50 times in a row then move on to one of the swordsmen by the gate stairway idol.
You’ll absorb a lot of parry timing that way. However, the two bosses you’re on will require different strategies than understanding basic combat.