An archetype-specific card, but a solid synergy piece/build-around in that archetype. Are you running a critical mass of Into the Fray, Relentless Assault, Moment of Triumph, and Hand Cannon? Possibly on a hero with built-in cards with overkill (Rocket's Particle Cannon, Thor's Hammer Throw), or which already synergize with some of those (Ms. Marvel's Embiggen)? Include Follow Through and go to town.

Example Thor deck: marvelcdb.com

I'm also intrigued in knowing how this will work with Wasp's Giant ability... Let's assume I have 2 Follow Through in play, and I have two engaged minions with 1 HP remaining each. If I attack them, using Hand Cannon's boost and deal 2 and 2 damage... this means that the villain will be dealt 6 damage, or I'll be able to trigger each Follow Through just once? (because it's just one attack)

In True-Solo tempo is often better than building a board and this card may struggle to justify its own cost. As player count increases though this card's value spikes massively. Every extra player is another encounter card per turn and thus higher minion odds. More minions = consistent excess damage targets, not to mention rush playstyles stop working. In these instances having a build out option that enables more healing, threat removal, and overkill damage is fantastic. At Solo I'd grade this card D due to short games and incredibly rare opportunities to utilize excess damage. At 2 I'd grade this at B, coinflip consistency unless intentionally playing minion heavy scenarios. At 3 it shoots up to A+ and at 4 S+, generally playable in every scenario and typically one of the best things you can play turn 1. If you had to choose between playing this and dropping an 8 damage Lunging Strike turn 1 this will always be worth more, with the opposite being true in Solo.