Mileena is sadly not a heavy hitter. Mix her juggle combos with some of her branchers. Don't expect to do moe than 30% damage though. She is pretty weak.
And please do not write the subject title all in caps. Thanks.

ooo, im the Mileena master!! Shes my favorite character, and common guys, leave her alone, shes not that weak lol....but i know a combo with her Sais
square square back square up triangle and soaring sai............i dunno if its that powerful, but its FUN! lol

thnks...but i play x box....but i kno the combos ur talkin about. thanks ill try that one. any other good hitters?

ok! Thats cool!

I think I tried that already.... lol.. is cool though..
ps. I like your Avatar EN.

The highest damage I have done with Mileena is 20% with her Sai's, for XBox its X,X,UP+Y,BACK+X(while your opponent is in the air),then X,X,UP+Y again(while your opponent is in the air).

Funny answered this on gamefaqs not too long ago
here's a link to the thread plenty of combos there.
http://boards.gamefaqs.com/gfaqs/genmessage.php?board=919922&topic=18300629
Best being the one i found in mian-chuan 4,1,4,4,3,cs hard to time hard to get it but it does work 9 hits 35%.