Multiple 0-0-0 Dartling Gunners are a good source of damage versus singular early MOABs, but they are inaccurate. Better still is to get a 2-2-0 Dartling Gunner to combat the MOAB, as it is a lot more cost-effective.
The main weaknesses of the Dartling Gunner are grouped bloons and leads, especially DDTs. Add support that counters grouped bloons, and add MIB to higher-level Dartling Gunners to damage DDTs. Slow down DDTs with MOAB Glue or Bloon Sabotage.
When trying to deal with fast rushes of Bloons, it is a good idea to buy the Faster Swivel upgrade. This allows the Dartling Gunner to rotate faster, as Dartling Gunner now has a cap on how fast it turns, a new feature of BTD6.
Try using a trio of Buckshot, Laser Cannon and Hydra Rocket Pods to maximize firepower. Each of them are useful in their own right, with Laser Cannon being good versus single-target bloons, Hydra Rocket Pods against grouped bloons, and Buckshot for burst damage.