Act 6.3.6 Boss (Captain America IW) is one of the annoying bosses that doesn’t take any damage unless the hit is critical and his balancing ability also makes it quite difficult to land critical hits. On top of that – His throws special attack in a sequence: Special 1 –> Special 2 –> and Special 3. There are a few champs that can handle, e.g. Stary, Ghost with Hood Synergy and Gwenpool. Any other champs will probably make this fight very expensive.
Here is a demo where Spiderman Stark-Enhanced Soloed him
Ronan vs Captain America IW Solo