I think another way would be to check your player equipment with a parallel event and do a quick change when you swap equipment. Obviously easier with plugins, but still doable without them.
Not a fan of enemies getting stronger/weaker depending on whether or not that are in the MC's party either, but you could have done a few things there to make him stronger, such as first turn immunity skill before the party can act. (Since he is betraying you and all (not that it is really a surprise, but he might still have a chance to cast before you can retaliate)). Just something to consider.
I suspect that since it is an enemy, the paralyze skill doesn't work completely properly without a plugin for your purpose and if you used a plugin, it could be that it stops the initiation, but not the resolution once it starts the casting.