Generally speaking in miniature wargaming terms the “alpha-strike” is a tactic where you build your army to unleash such a powerful attack in a single turn that the opponent is either completely destroyed or crippled in such a way that they can’t effetely continue over the coming game turns- either way assuring the win. In some game systems it is very easy to build the alpha-strike while in others more difficult- so what about in Battletech?
While ones tactical thoughts could turn to a support lance of all long-range missile mechs, the alpha strike I feel is better delivered up close since the game mechanics deal with random hit locations (at least for early era tech.)
In keeping with the idea of an alpha strike being delivered to a mech to destroy it, then one would want to select a mech that can mount and fire a number of medium to heavy damage weapons at once. Firing multiple weapons at once means more of a chance to hit an already damaged or weakened spot from either the first few hits, or damaged taken over the turns on the way in.
As a tactical example the Battlemaster is the walking example of this- as it moves across the battlefield it uses its PPC to soften up the target, aided by a medium laser and SM6 missile salvo, and then when close almost point blank (using its heavy armor to get there), it opens up with the full six medium lasers and the machine guns.
That is the alpha-strike against a light, medium, or even damaged heavy meach.