How to deal with Turtle's shield
The main weakness of Turtle's shield are explosives, like Nader, Javelin, etc. The shield has a bullet damage modifier (0.9), but I think explosives can do full damage to it (can't confirm this). Desconsidering explosives, Rhino it's the best merc to take down the shield quickly, he has a very high DPS, and his low accuracy it's not a problem since shield's hitbox is large.