some tips for choosing main:

fireM - for aoe and debuff route (sleep/shield/control)

lightningM - blitz em XD

waterM - for turtle team (dem heals/immortal team) or plain nuke+poison

windM - for clone/team buff+cleanse with aoe

earthM - shield bro, or use the 2x mystery and make every ninja in your team attack twice

you can search in forum for the exact team for each main with the suitable ninja ..

hope that helps ..