With double artemis, youre able to disable their guns, but much like double hwacha you'll have a tough time cracking armor due to the explosive/shatter damage (which are both bad against armor). That, combined with the fact that the Galleon can soak a ton of damage (it has way more armor and hull than a pyra), means that unless you carefully manage your positioning, you're gonna have a bad time.
You shouldn't expect to kill much of anything with double artemis unless you coordinate with your ally to bring down their armor. In this case, your ally better bring at least a merc or gat. Double artemis can't crack armor well, but the explosive damage means that it can hurt permahull decently.
EDIT: Also, the 'easily destroyed' thing he was referring to was the hwacha, not the galleon. with a merc, artemis, or gat, you shouldn't have much problem taking out the guns.