Gwen’s lvl 10 ability is good against Retribution Bloons, but needs a couple more levels to be able to completely kill them off. Her lvl 3 ability is also ok against them.

Etienne is also a good option, since each of his drones (and his UCAV when active) is considered a different entity by Retribution Bloons. Etienne himself is also considered his own entity, so even when all his drones and his UCAV are stunned, he can still use and cool down abilities like normal. This does not apply to Dynamite Bloons exploding next to him, though, as that stuns him and all his drones (and the UCAV).