If you have the stats, but are losing on nightmade difficulty, you need to adjust something.
Many have mentioned different characters, which is a no brainer, but I'd also suggest having multiple pets.
Currently, I roll with:
A Huntress Guardians for my Squire/Apprentice Builder.
I have an awesome Trans Genie (for tower upgrades) and Mythical Djinnlett (keeps invis/ps going) for the later waves for my Huntress.
Those will make all the difference, trust me.
Interesting catch 22 there. My pets lack but to get new pets I need to clear NM maps. :)