For the sake of the argument, I'm making Cap PL10 and having the SHIELD agents be PL5 minions who use the "Soldier" minion model (one of the more powerful minion models, good for well-trained soldiers).
Cap: trade off attack/damage and defense/toughness
Attack:
+14 (max +6 damage, and I'm assuming that it's max'd out with some combination of Strength and weapon(Shield))
Defense:
+12 including shield (max +8 toughness with Chainmail, Constitution)
In terms of powers, I give Cap Deflection for his shield, dropping it to a reaction and putting points in it to bring it up to his Defense level. So his defense is really d20+12 (minimum 22).
He's surrounded by SHIELD agents, who attack at +5 for +5 damage (assault rifle).
I see Cap as mowing through those guys, given the rules for minions:
- Cap needs to hit each minion (and have that minion fail a Toughness save) once to knock him unconscious
- Cap can take 10 on attacks against minions
- Minions don't get the benefit of the Impossible Toughness Save rule (see below)
- With two ranks of Takedown Attack (M&M's version of Cleave), Cap can take a 5-foot step after each bad guy he takes down, up to his normal movement rate
Cap is pretty much your basic feat-based combat hero, so he has Power Attack, Defensive Attack, two ranks of Takedown Attack, and a whole bunch of other stuff that I'm not writing here.
My interpretation of the fight would be:
Cap puts 5 points into Power Attack, dropping his attack to +9 and increasing his damage to +11.
Cap hits the first bad guy (taking 10, auto-hit), and the SHIELD agent has to make a Toughness save (d20+5) against a DC of 26 (15+11). The minion cannot make the save, even on a 20 (the "Impossible Toughness Save" rule does not apply to minions), so he's auto-knocked-out, and Cap takes a 5-foot-step and auto-hits (and auto-knocks-out) the NEXT SHIELD agent... and so on.
Assuming that there are an even dozen SHIELD agents, Cap can reasonably finish them off in three rounds. If he's smart, he'd be using Defensive Attack (M&M Expertise) up to +4 as well so that his defense jumps from 22 to 26 (he can't take it to 27, because then his attack bonus is low enough that he can't hit by takeing 10). Putting his Defense at 26 means that the bad guys need a natural 20 to hit him (or that they need to use Aid Another to help improve their chances of hitting).
A hit will hurt Cap, since he's only saving at d20+8 against a DC of 20. I'd expect him to take a bruise or two just based on the bad guys rolling well or using Aid Another. Since he's taking down three or four enemies per round, though, he doesn't have to worry about this for long. If they were behind cover and he couldn't get to them, then yes, Cap would be in serious danger of getting whittled down -- but in a straight-up fight, the SHIELD agents are about 20 seconds of hard work.