Taskmaster is the boss in Captain America's story. He is known to use his photographic memory to counter the moves of opponents and then to use them against them. He has prepared for this fight by studying and learning up on Cap. However Captain America beats him through the use of improvisation.
Taskmaster is faced relatively early on in the story at level 25, so you may need to go back and train for a few more levels. Taskmaster relies on easy to dodge punches, hard-hitting sword strikes, and a long range bow with arrow. Taskmaster has a unique ability in that, when stunned, you can perform each heroic combo (3 strikes in the same direction with the 3rd hitting harder) he has a chance to block it next time. The next time you try to perform the combo, there is a chance that Taskmaster's phtographic memory kicks in and blocks your attack and removes the stun. This only applies to combos you've already used and it doesn't always happen.