Well, should it???
I think it'd be a nice balance; in order to use this instant kill melee attack [i](BULLSHIT)[/i], then Stormfist should be charged, and it should be expended to use this ability. Simple. Limits the amount of ridiculous instant kills that Titans can achieve.
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Or perhaps we just replace Stormfist with Shouldercharge, and replace Shouldercharge with another perk?