Easiest place to kill is in Brimhaven Dungeon, just take the north path when inside (Axe needed), and go up the ladder, then past the fire giants.
They can hit a bit, but the biggest problem is the Wild dogs that frequently attack you.
They usually just drop coins, but sometimes drop charms. They rarely drop deaths, chaos, half keys, and rune full helms.
When training here, not much attention is needed, just keep an eye on health.
greater demon greaters demons