Cap and Widow fought ground troops and among ALL of them, Cap is the single best hand to hand combatant. Tony was above ground, fighting stronger guys. And while these aliens are menacing in number and weaponry:
a)Captain America's shield is INDESTRUCTIBLE. Nothing they can do to it would harm it in anyway. If Thor can't do it, the aliens can't.
b)That indestructible shield? As shown, Cap can wield it like no one else can, so almost any blast they throw at him him will be deflected, which means the troops have to go into hand to had combat with, again, the single most skilled hand to hand combatant the team has. And since none of the aliens were shown to have super-strength...yeah, Cap wins.
C)It wasn't that easy of a time for Cap, as he got tore up and was exhausted by the end of the battle. THEY ALL WERE.
d) They all had jobs, as given to them by Cap. As stated earlier, what Tony was dealing with in the sky was different than what Cap and Widow had to deal with. Tony was dealing with more heavy artillery essentially, because his armor could take it, and he also took more than a few hits. Tony was taking some serious hits from more powerful sources, but that's what he could do.

As for Thor Hulk, I fail to see any issue there. What's the problem with Thor being able to fight the Hulk? He's a god...and that giant alien machine creature, while large, doesn't move anywhere near fast enough to be able to stop the Hulk in anyway, it's size is both an advantage and a disadvantage. Thor on the other hand is far more able to deal with Hulk in size and strength. It's fairly cut and dry...