Depends on what you want to do with him. Merlin is generally good with him for both Crits and NP usage, if you want more crits add jack or MHXA, if you want more NP buster memes add someone like Shakespeare. If it's for a long challenge quest, add a taunter like Mash. Don't mix with Riders or Archers if you want him to get Crit Stars reliably.
Who are the best servants to pair with Karna?
Merlin for massive buster and crit bonuses, ofc.
Waver, the ultimate generalist.
Hans for free stars, more crit bonuses, and good spammeable support NP
Leonidas, fellow lancer, taunter with 3 turns 25% buster up CD of 5.
Caesar, attack up (the bonus he does not have), even more NP dmg up, even more stragen and crit ups.
Anyone who can give AOE attack up, as this is the only bonus he does not have, to multiplicate with Mana Burst Flames (high Charisma).