Dagannoth guardians do not drop charms, however, they do drop level 2 clue scrolls fairly often. If you use the Protect from Melee prayer or the Deflect Melee curse, the Dagannoth guardian will use its ranged attack until it dies or until you put a Missile prayer on, at that point, it will go back to its melee attack. You should also note, that you cannot use a dwarf multicannon in the dungeon. You'll get a warning that it's too dark to work properly.