Not sure on what job it should be, but perhaps SCH since it borrows a bit from the FFXI SCH.

Spell: Helix
Places a single stack of Helix(DoT) on the target.
Duration: 30 seconds, recast: 1 minute
Recasting the spell on a target overwrites current Helix.

Skill: Empower
Grants 1 stack of Helix to a target currently under the effect of the spell, while cutting the remaining duration in half.

Skill: Extend
Removes Bio from the target and adds remaining duration to Helix' duration.

Basic flow would be to apply Helix, then Bio, use Empower for the second stack, then Extend to remove Bio and extend the duration of Helix.
Helix would have a cap on stacks(perhaps 5) and is something you build up over the fight and then maintain.
Cooldown for Empower and Extend would be short'ish, but not spamable(perhaps 10-15 seconds).