It would depend on your str lvls? i would recomend doing crabs if your str is high with a maul, or a SC staff so you can get the 2k xp. the SC staffs are really worth it and would help you train. If not the seagulls are VERY good for an obby mauler )
i also have a f2p mauler. and the PERFECT spot is giant rats. south of varok and north of the cow patch next to al kahrid. the have 530 hp i think and hardley ever hit. there are loads of them and hardley anyone goes there. the only thing bad is that it is click intensive. as you one hit most of them. but if you are going to f2p train thats what you can expect.
anythign with a high hp would be bad for a 1 atk/def pure.