The fleet:
The method:
Deploy the weaponless monitor, ISS Bastion. Fortress shield up. Gather all enemies to me and tank them.
Deploy 1-3 medusas depending on size and strength of enemy forces.
Wait for medusas to arrive together. 90% of the time they choose the same target on the periphery of my tanking circle since they deployed simultaneously and begin pelting it with heavy blasters (formerly pulse lasers + antimatter blasters) until it is dead.
Weave around enemies to position them in a line so they won't fire on me if necessary, while staying close enough to keep their attention, and toggle fortress shield on and off to regain flux. This step is unnecessary against forces that don't have multiple cruisers or a capital ship + backup.
Repeat until medusas have picked off each enemy.
Win.
If anything escaped, the tempest chases it down and finishes it with superior speed and extreme prejudice.
If any ship remains intact, the venerable ISS Sudoku shuttles on over for a suicide marine rush, if desired.
This works well. I have even successfully taken on fleets with an onslaught + cruisers and destroyers and more, albeit with significant damage done to the poor, silly AI medusas. I met my match with this setup in the form of a fleet with two onslaughts deployed simultaneously, as my poor monitor could not keep the attention of enough of their combined guns to save stupid medusas from lowering their shields instead of backing off, and getting exploded.