My current spire build is:
Merc top front, 2x Artemis left and right side. Hell hound for the main gun. I use the hellhound as a backup weapon while mostly relying on the Merc, Artemis, Artemis combo to take out my opponent at long range. The engineers take burst while the gunner takes charged heavy and burst. Most of the game the gunner sits on the merc putting holes in the other ship's armor while the engines shoot the Artemis. If the enemy gets too close, the gunner jumps down to to the hell hound and takes out the balloon, giving the spire some breathing room to run to his ally. When under fire both engineers will be at the hull and balloon so the cannonade is used more for getting out of a jam than actually killing.
This build has won and lost about the same number of matches. It is weak against squids and blender fish, but strong against pyramidions, galleons and junkers, provided it does not get ambushed. Also with the entire crew shooting the hull occasionally gets neglected. This build requires a very observant engineer and an obedient gunner.