Looking forward to make this card valuable. My first thought is, put it into play, play Heimdall to rearrange the top of the encounter deck and then BOOM! Also, Scarlet Witch should be able to add a little magic to make things interesting...
Hero Action (attack): Discard Pulse Grenade and choose an enemy → discard the top 2 cards of the encounter deck. Deal 1 damage to the chosen enemy for each boost icon discarded this way.
On average, you'll get about four damage out of Pulse Grenade, with a practical maximum of six. Haymaker is nearly unplayable with only three damage at this cost, but four damage is ok, and 5+ damage is up there with some of the better aspect and hero attacks. If you ever get 0-2 damage though, you're gonna feel real bad. There are some Weapon synergies (Fusillade, Mean Swing, etc) but those are fairly minor.
The big value though is that you get your damage on a delay. Most villains are significantly more threatening in their later phases, and/or summon a bunch of extra problems at phase transition. Setting down a grenade or two when you have things under control in an early phase will make your life better later. If you have all three in front of you, it's entirely possible you can just delete the last phase entirely - many villains have fewer than 20 hit points per player. Three grenades is an average of 12 damage, and possibly as much as 18; combine that with an attack from your hero and a played attack card, and you might well just win immediately.