It will take a gem or two.
So 4 turns with a maxed Merry, 6 with a x1.2 one (8 with the dinghy you're using there
). That's a worst case scenario so it can take one less turn if you get more orbs in the coby turns.
Damage is not the only issue with the team, the HP is pretty low too. In its current state your team can only take Helmeppo's pre-emptive, 2 hits from Garp (so you get to attack him 3 times) and ONE hit from the turtle. Basically you can't stall for long on the turtle stage (unless you get meat orbs) so Robin and Broggy's specials may or may not be ready depending on how many skill ups you got on them.
2x Nami teams without a Vista sub really need that x1.5 Merry and nearly maxed units.
Edit: Do you have a Gear 3? He might help.
Thanks. You added me this week, I'm "Korial Mads".