Chara

Chara is the second final main boss in the game that doesn't need a reset.

Strategy:
You need the locket and real knife to do this, Error hat and error knife is

even better but they are harder to get. Just run around her and jumping on her.

I recommend food because this is a difficult fight

Strategy (hardmode) :
Immediately, the moment you load into this fight's hard mode variant, you will notice a horrifying truth: You aren't allowed to use items in the hard mode version. So how do we beat the fight? Well, personally, I recommend the 'Error Hat' and 'Error' combo. This is because it heals you everytime you hit chara, it also gives you a 2% damage boost per hit too. Or, you can also just use kindness soul and hope for the best.

Trivia:
Chara is the second last boss in boss rush, the first being gaster

although being before gaster, Chara is a lot stronger.