Here is my recommend team setup. Hawkeye, Black Widow and Captain America.
It is best to get this as early as possible since it will be much easier and you wont be overpowered. What you do is choose one of the levels in the first area. It should be the first area so you do not have any team gems in play. Second, I recommend choosing the highest level of enemies you can handle, just so you last longer.
The strategy is to get Captain America to 19 yellow gems. His special will allow you to place 3 yellow gems anywhere on the board. Now you can combine this with Hawkeye's Purple ability which will spawn one combo gem. Since combo gems use any color, you can use that in combination with Captain America's Yellow ability to get 8 gems. Remember, the last gem you place with Captain America needs to finish the chain of 8, or else you will set 7 or so off.
Black Widow is there just as a backup for her stun if you need it/have it. Just in case you are in a situation where Captain America is almost dead.