Ia m going to do the " Prisoners of War " Quest to rescue some guy from the thalmor. Myself and Lydia go in guns a blazing with out the help of the mercenary We do pretty well I am dual casting fire i look around Lydia has bitten the dust i get my butt handed to me at lvl 20. Whats a good replacement for Lydia?

she's one of the best followers imo.

you gave her a shield//health potions right?

Well with UFO mod it makes them much more realistic , If I don't protect them they all die

Any follower is a good replacement for her. If you're looking for another heavy armored follower, I suggest http://www.uesp.net/wiki/Skyrim:Vorstag. He has pretty good combat skills.

I am thinking of going with http://www.uesp.net/wiki/Skyrim:Vorstag he seems like a suitable replacement hopefully he fits into Lydias armor when I go collecting it and turn him into my next mule to carry my crap.