What should I choose for a fire killer Sam, Elin or Louis to train

I'd recommend neither of them. I will explain you why:

Elin - she is absolutely weak at the current state of the game and rather useless, she cannot be hotfired (for you as a fire troublesome as a pet) and she cannot wear any other weapons else her stronger skills (besides the starter-skill that she has before she gets gun/chip) cannot be used. It was planned by the game creators for Elin to get Terrafication-Skill in later versions. Since this never happened, which would have made her really useful, I recomment to not train Elin as a serious main-pet anymore. You can use her as a second-pet for pretty-reasons, or train her for fun on a char you don't really use.

Both Sam and Louis are water-pets and strength-based. Louis only has attack-skills, but rather weak ones. Sam isn't strong either when not fed, the only useful thing about him is the water shield, in case you plan to go to big PK events like Interserver-PK and you fight against many highly fed mage-pets (mostly Niss/Fred). It could have kind of an effect using water shild against those fed Nisses... but you should really not take a Fire-Killer to PK, so Sam is useless for you.

If you already plan to grind-train, use a Frederico or Niss that you can hf, or at least a stronger ATK-pet like Yanlinh ("Victoria") or a supporter (e.g. for Quests) like Xaolan or Shasha.
If I were you, I'd forget about all the three pets that you mentioned.