Slayer (which is Killer) fills only when an enemy is defeated. Warrior fills per attack.

Chances are, you should learn Warrior for Tidus, Auron, Wakka, and Kimahri by Djose. Alternatively, Wakka may also learn Tactician (Overdrive fills based on status infliction). Yuna should learn Healer by then also, and Lulu might learn Warrior, but since she does mangic damage, it is unlikely. All of your character may or may not learn Ally (filling based on turns) by Djose, also.

If you're learning Sufferer/Coward within normal game limits, then you need to stop playing games altogether.