you can set up a Nightmare Zone scenario where you can AFK train for 20 minutes at a time. Using an imbued magic shortbow and amethyst or rune arrows is the usual way to go. You'll also need some super ranging and absorption potions...
Dagannoth Prime, being weak to Ranged attacks, can be effectively dealt with using Ranged weaponry. Recommended Ranged weapons include the Zaryte, Armadyl, Dragon hunter, dragon, and rune crossbows. The Toxic blowpipe is also a solid choice, particularly on Slayer tasks. If you have the resource...