Whats the eastiest method to kill them? I have dragon legs, rune plate, mask, barrows gloves, crystal shield, whip, and guthans. Should I bring food noted and bank, or heal guthans? All suggestions welcome

Cannons are great for Dagganoth tasks, rips through them very quickly. If you don't have that, Guthans and a whip should be fine for them.

wear guthans full time and switch to spear when needed?

Yeah, but wear the black mask over Guthans Helm when not healing, it's attack bonuses are very useful.

melee with guthans is fine, but cannon will own all the way

don't have anymore. I will be fine with pure attack experience :)