From Mechwarrior Living Legends Wiki
An alpha strike is an attack where a 'Mech fires all its weapons simultaneously.
Alpha strikes are very effective because they tightly concentrate damage, leading to heavy damage to or destruction of some components. Some 'Mechs are designed to make use of alpha strikes. Notably, the Puma variant C is capable of crippling any light 'Mech with few well-placed MBL alpha strikes, as is the 2 CGauss Vulture A, the Madcat MKII A or any variant of Novacat.
Any 'Mech can unleash the alpha strike, but it's not always an effective tactic. Firstly, the 'Mech should be wielding weapons with similar characteristics, for example only UACs, only Lasers, or only PPCs/Gauss (which behave similarly) -- these setups allow all weapons will hit the same or very close spot of the 'Mech, inflicting concentrated damage (though weapons can be grouped more freely when shooting slow targets at very close range). Secondly, some alpha strike configurations will cause severe overheating, which will damage one's own 'Mech and threaten a shut down or self-destruction. Examples of 'Mechs with hazardous alpha strikes include the Madcat MKII C (especially when jump jetting), and variants of the energy weapon-heavy Awesome and Novacat 'Mechs, some of which can peg the core temperature gauge in a single alpha strike.
There isn't much defense against an alpha strike, other than attempting to make the enemy pilot miss the shot, or shielding vulnerable components by turning or using terrain. However, alpha strikes generate severe amounts of heat in a short time (especially when it's based on energy weapons), so it may help to raise his heat with PPC shots or (preferably) Flamers. Further, if the opportunity arises, destroying or heavily damaging the back armor of an alpha-striking mech will force the pilot to be more conservative with heat or risk self-killing, since incurring heat damage after the back armor is destroyed will instantly destroy the mech.