Strength of similarly levelled Pokémon

I found this mod yesterday and thought it was really good. I like the designs, and the animations are awesome. However, beating any Pokémon at all is so unnecessarily hard!

I was fighting a level 14 Pokémon that had a type disadvantage against my level 13 Snivy. I also had a level 10 Igglybuff as backup. However, I lost, BADLY. My tackles would do, like 10% damage at the most on my Snivy, whereas the opposing Pokémon's did like, 1/3 of my health in one. This was with no buffs or debuffs, btw. What is going on? Do I just need to target lower level Pokémon? Because it seems like I'd only be able to beat ones who are significantly lower level.