I've been running a game now for about a year and a half now, and challenging the group really isn't that important of a thing. Make the game fun, entertaining, and give different people their chances to shine. I've learned in that time that some of the "easy" encounters will nearly wipe your group, and some of the "tough" encounters get rolled over like they are nothing. If the group comes up with an idea that you didn't think of, then cool. They are thinking about the game, and trying to solve problems. Sometimes your players will engage in paranoid conjecture, and their ideas will turn out to be much better than what you had planned. Take the ball and roll with it.
Now, for practical things. If the targets consist of 4-5 people, and one of them is armored up so much that I don't think my shot will hurt him, then I am not shooting at him. Pick a softer target. NPCs don't have to be stupid. They don't have to waste shots shooting at the thing they cannot hurt, shoot the things they can hurt. As mentioned before, there is no "aggro" mechanic forcing enemies to stupidly waste shots on the most armored target in the field. The tank has very high damage resistance, but enemies should realize that real soon (if not visibly obvious, then it's obvious after the first Barrett round doesn't drop the target) and react accordingly.
Falling Damage. Levitate Spells can be resisted, but if a mage levitates a heavily armored target, and floats him 20 meters into the air before dismissing, 20P -4AP is going to leave a mark
Shoot the air tank, then have a spirit of Air engulf the target. They can resist the damage without armor as they begin suffocating.
Environmental effects. Surround the tank with smoke grenades to reduce their offensive capability, and then just ignore them. Tanks can only tank, when someone is giving them something to tank. Like above, if the NPCs focus on softer targets and leave the tank alone with diminished offensive capability, then the bad guys are succeeding.
Again, I will agree with others. Take the group out of combat. Maybe they are being interviewed by a Johnson or contact, and he doesn't want the face doing all the talking. Maybe he wants to interact with each of the PCs directly, or separately. Have situations where multiple people need to socially engage in targets simultaneously. Imagine if the group is caught sneaking through some place and the guards wonder why the others are being so quiet. They should speak for themselves and answer the questions.
Looking at other posts here, it seems that halflingmage has covered pretty much most of the important parts. I specially like the punch him in the girlfriend (Gamer's Tavern? Or was that Fear the Boot? I forget). When the villains didn't have kryptonite, going after innocent bystanders sure did get to Superman.