Okay, let's continue this! I have faith in you little bugger. Let's start things off with RNG hacks, and go for Mirror Shot, follow up with Giga Drain and Flash Cannon, to see how much the Giga Drain does.
A few subs as well, if he doesn't use taunt or blocks the use of leech seed in any way, go for Leech Seed instead of Flash Cannon, also, if your health is getting critically low (~15%) go for Explosion! Take this Snorlax with you! (Or at least a little bit of it's health, that's fine too. )

Again, sorry for me sounding pissed off, I was certainly not! (okay maybe a lil bit) A gym battle should be a hard challenge, and I'm happy it is one! (: