No monster (other than Bork) drops any charm 100% of the time. I would highly suggest Waterfiends, they aren't as much as a hassle as them seem! Don't they need anti-fire pots and pray pots? Actually, now looking at your stats a bit closer I would just stay at Dust Devils for now, and go to the Waterfiends at around 68 summoning. (for the "mighty" Bunyip) You should bring prayer pots to protect against the Ranged Attack of the fiends. Also, before each trip, drink only a dose of antifire potion and run to the larger spot (across the pond) with Mage Prayer activated. So just take an antifire pot, the other essentials and then just take prayer pots?