Just start out on the low monsters and just quickly move up to bigger things since you are a f2p try the hill giants or something (sorry if those aren't f2p I haven't been on in a few months and I can't remember).
Go to Minotaurs on the first level of the Security stronghold. Never crowded + lots of monsters. ( Fight the level 27's, also drop alot of iron arrows ).
Go to flesh crawlers on the second level of the StrongHold of Security. They have 25 hp (I think) and auto attack you. Basically like killing half a rock crab... Not bad for HP eh?