Pokemon

Torterra: Grass/Ground

Infernape: Fire/Fighting

Empoleon: Water/Steel

Fire counters grass, which counters water, which counters fire.

that is kind of obvious

Ground beats fighting

fighting beats steel

Apparently ground also beats steel

But torterra grass/ground has the worst base stats.

You can battle with them to get exp points

Or you could level up the friendship to make it do better in battles.

Using certain items on them

trading them to another player

trading them to another player while holding an item

Leveling up the friendship

Those are the other common ways to evolve a pokemon