The bomb tower has an upgrade that does 10 times more damage to MOAB class bloons (MOAB mauler). It's not clear how many hits from that will pop a ZOMG.
Also we have the Cripple MOAB upgrade for the sniper. Again, I do not know how many hits from that will pop a ZOMG.
Anything else?
Some say that a Dragon's Breath apprentices are good against it. If so, how? Does that do extra damage against ZOMGs? What?
Also, the Sun God may be good against ZOMGs, but why?
I notice that Rays of Doom are good against MOABs, but not that good against unclustered ZOMGs.
Best Answer
As far as I'm aware, a fire mage has nothing that would allow it to be extra effective against ZOMG bloons, other than cleaning up after they've been destroyed. A Sun God is good against a ZOMG purely because of its raw damage output, but it does need assistance from other towers. The Sniper Monkey 4/2 temporarily stops a ZOMG from moving, as well as other blimp bloons.
Here are a few towers other towers that have special abilities against ZOMG:
An upgraded Bloonchipper (4/2) is able to suck in the ZOMG and then eject after sawing at it for a while.
A ninja monkey with the 'Sabotage Supply Lines' (2/4) ability is able to slow it significantly.
A Technological Terror (Super Monkey 2/4) has an AOE ability that deals 1000 damage to ZOMG bloons.
A Spike Factory (2/4) is designed to be effective against all blimp bloons, causing extra damage.
A Monkey Sub (2/4) has the First Strike Capabilty that destroys up to 1 ZOMG in one shot.