Medics don't make troops invincible. They just make them more resistant to AoE defenses as far as my opinion goes.
Before I start off with the presentation of proof, I would have to say I have had those times where my Tmed lost some Tanks while hit by only AoE defenses. Yes, sometimes it involved me going up against the multi-Rocket Launcher base, and sometimes against Dr. Terror where even a massed amount of Mortars, Machine Guns and Rocket Launchers bested my Tanks and Medics, at least beating 1 or 2 Tanks, or even defeating my whole army.
So this guy came up with Grenadiers and Medics and decided to attack my Resource Base. Yes, he's crazy going up against a level 60 base, that is boosted up with 24% Building Health on all maxed out defenses. Sure a Rocket Launcher and a Mortar. That's probably gonna do nothing, right? Wrong. With the first salvo of the Rocket Launcher and the first Mortar shell, three Grenadiers go down.
Another two and a Medic after the second Mortar shell, and as he orders the retreat, one more Gren and one more Medic go down with the third Mortar shell.
Damage done: Not even close.
Lesson learnt: Never underestimate maxed Mortars when they're combined with a RL :P