Can I kill Divining Shells to get a unit +2 during a showdown ? Can it be played like an action to give my unit +2 ?

Yes — you can activate Divining Shells' ability during a showdown to give a unit +2 Might, because that activated ability carries the [Action] tag (Rule 806.1.c.2), which lets it be activated in a Showdown Open State on any player's turn.

One correction on wording: you're not "playing it like an action" — it's a Gear activated ability, not an Action spell. It still goes onto the chain as an ability (and can be responded to or countered), and the "Kill this, [T]" is a cost, so the Shells are already in the trash before it resolves. You only get one use out of it.

Timing matters: it has to be activated while the showdown is still in an Open State — you cannot activate it during the Combat Damage Step, so you must give the +2 before the showdown closes and damage is assigned.