I was using a double artemis pyramidion setup (with mortar and carronade on the side) and got nowhere in my long range attacks on that Galleon. The whole match was ridiculous. Easily destroyed, huh?
Remove one of the Artemis and instead put a Mercury in its place.
There, I fixed your ship.
No, seriously if you have an even remotely competent gunner by the time you get at medim range the Galleon won't have a single weapon still functioning and from there you can basically keep him locked rebuilding components while you damage the Hull. It's a tried and tested build.
And double Manticore on one broadside is not that strong at all, whenever I see an opponent going with that loadout I think to myself "easy points", because that means no Flak cannon that kills me in 2 hits when the armor goes down, no Lumberjack to keep my ship pernanently at ground level and no Lochnagar Hellhound to strip my armor in one hit.
Sure if I make a mistake they might be able to disable my ship but it's hardly one of the better setups for the Galleon. One Manticore is enough.