If i could beat it with an Eclipse Axe using those advices, i bet you could do
it with a mace too...
But if you use the Arquebus, i'd recommend to go for the barrels underneath
it, as they go boom, and usually destroys it.
I dont know if a shot will blow them up though, as im a single-weapon user...
Or just grab your best hammer and follow the first advice >_>
An alternative would be to play the arena, and beat the Runic-sword guy until
you get a small red gem. Not as good as a large one, but it works...