Exterminator is an achievement in Bloons TD 4 IOS awarded for popping 1,000,000 Bloons in total. Doing missions that involve lots of M.O.A.B.s and B.F.B.s can make this easy to get (like the mission where the player has to pop 40 B.F.B.s) or going into Freeplay can make this easier to get.