As an Arcanist, it's extremely difficult to participate in a kill.
I'm level 32, so I've participated in many "kill X mobs" quests and when competing with DoW classes the mob often will die before Ruin even finishes casting -- let alone enough repeated actions to qualify for credit.
Often, I've had to resort to tagging mobs with Bio and letting Topaz Carbuncle tank them as others kill them.
This is, of course, something easily remedied by forming a party but rare is the case that others accept the invite.
I haven't a suggestion short of making Ruin instant cast or Bio deal an initial amount of damage.