So say My LE Shazam! runs right next to Terrax.
Now say Terrax misses or whatever. Then LE Shazam
hits Terrax using Exploit weakness. Does Terrax
still roll the 5 or 6 for Impervious since
Exploit Weakness only reduces damage dealt?

Exploit Weakness ignores all damage reducing powers. Impervious reduces damage. Exploit Weankess ignores Impervious

i think EW ignores all powers that reduce damage, like PB. i could be wrong, but i'm fairly sure that's how it works. so, no there wouldn't be a roll for impervious as the power is "ignored"

But super senses works?

Yes, because super senses is not a damage reducing power.

Senses dodge the attack.
Imperv reduces the damage.

EW and PBlast ignore damage reduction powers.

So EW and Pblast go through Toughness, Invuln and Imperv, but can be dodged by Super Senses. Also, Skrull TA and Shapeshift may force the EW and Pblaster to look elsewhere for their action.