One of the Elite Four had a Skarmory. I tried many times to defeat it before I realised that Steel was weak against Fire. Even then, it took me a lot of hassle to KO the bird. Probably because I was using that sluggish oaf Torkoal.
Kyogre uses Hydro Pump... the attack missed. Ninetails uses Flamethrower... it's not very effective. Kyogre uses Hydro Pump... it's super-effective! Ninetails has fainted. ~xFirefoxgoddessx is about to use Ninetails! Will Kitty switch Pokemon? NO
Ninetales used Sunny Day... The sunlight got bright! Kyogre uses Lock-On... Kyogre took aim at Ninetails! Ninetails used Solar Beam... (100 HP damage max) Kyogre used Hydro Pump... It's super-effective!
It's never likely that a nonlegendary will win a battle against a Legendary Pokemon of its weakness, assuming same levels. Although I'm not sure where you're getting the 100 HP damage max.
You seem to be getting a kick out of going thru this forum and offering strategies to counter everyone's choices. Fact is, all Pokemon have a weakness--that's how the game is designed--and I'm sure that everyone here is aware of that. What's your game...?