Borough Backup

Whenever a creature you control attacks alone, investigate. (Create a Clue token. It's an artifact with "2, Sacrifice this token: Draw a card.")

Whenever Agent Maria Hill becomes tapped to pay a teamwork cost, put a +1/+1 counter on her and draw a card.

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Vigilance T: Put a +1/+1 counter on each other Hero you control.

Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn.

Flash Heroes you control get +2/+2. At the beginning of each end step, if you attacked with a Hero this turn or a Hero entered the battlefield under your control this turn, draw a card.

Lifelink Power-up — 4W: Put two +1/+1 counters on this creature. (Activate each power-up ability only once. Reduce the cost by its mana cost if it entered this turn.)

First strike Captain America enters with a shield counter on him. (If he would be dealt damage or destroyed, remove a shield counter from him instead.) As long as Captain America has a shield counter on him, you and other Heroes you control have hexproof.