Wow, almost 4 years and nobody's mentioned Focus Energy? I have lost all faith in humanity...
Preparation
First off, you're gonna want to get as many Oran Berries as you can, Reviver Seeds too if you can find any.
The Fight
First turn, you're going to use Focus Energy. Then, when it approaches you, spam Ember. Focus Energy is quite literally broken in this game, and guaranteed crit Embers will do a lot more than a normal Ember and win you the battle a lot faster. When Focus Energy ends, use it again and continue to spam Ember.
As for Squirtle, it's going to spam Bubble (or Water Gun, but Bubble's chance to lower speed could help, although it doesn't really matter), since Squirtle can't do too much in this fight.
Moral of the story: Use Focus Energy. It's broken. Keep Focus Energy. It's broken.