my mage lvl is 55 and i wanted to know what will be a better way to get mage xp --> fire bolt or crumble undead....help plz

it has been proven that crumble the undead gives more xp per cast but it can be quite annoying to do over and over unlike fire bolt which can auto-cast. More important is which monster you train on but really as you are 55magic i suggest you high alch. It's very good xp really (if p2p, if not go with crumble the undead on skeletons under varrock).

you can alch in f2p also, there are ways to not lose too much money.

i asked about the attacking spells becuz i have 3.6k chaos runes...and very few nature runes but 10x alot fot the tips

if you have the cash, alching green d-hide bodies in f2p is really good, and you don't lose much. Fire bolt with those chaos is easier than casting crumble undead, though members have access to the family crest gauntlets.