Since the immortal still shuts down most of Terran mech, how about the following tweak:
Change the immortals shields so that there is a recharge delay every time they are triggered by a high damage attack. Let's say that any attack over forty or fifty damage will trigger the immortals shields, but then there's a .3 or .5 delay recharging time.
I also propose reducing the bonus damage and increasing the base damage. Or just reducing the bonus damage while decreasing cost and/or build time.
This way it's not going to be three immortals clearing out thirty tanks (exaggerating, before some literal minded person attacks this statement) anymore, making mech that much more viable again.
Whaddya think?