You could use concussion hits, you'd just have to impose a hefty number... and it should be a percentage, not a flat number.
Every time he uses his 'super strike' he loses 25% of his overall (not current) concussion hits. This would have the effect of not only making him more easy to knock out in battle, but also giving him a temporary -10 to all actions (due to damage) as suggested by Vroomfogle.
Further, you could also use a self discipline skill to 'control' the power drain. Something like: when the PC wants to use his super strike he rolls, using this skill's bonus and 'desire' modifiers like adrenal maneuvers. You could make it a simple success or failure using the static maneuver table, or more dynamic and use a column from the Movement Manuver Table T-4.1, perhaps VERY HARD, using this table could be used to reduce the damage taken from the super-strike.. Total falure (on either table) could result in no bonus to damage, and still the loss of life-force.
If you use the skill to reduce the damage you might want to up the 'baseline' damage to 50% or something like that.. so devloping the skill would be a priority using a 'super strike'.. there should also be a minimum damage taken, so even a PC with tons of ranks in the skill can't 'super strike' without losing life.
If you want to limit the uses per month without doing something so contrived as 'twice per month' you could track the 'life drain' concussion hits separately from regular hits. These hits might recover at 1/10 or 1/100 the normal rate (or somewhere in between).. this would make it so the PC could push himself and strike a number of times in a battle, but then be weak for months; or focus on recovery between missions and get to strike more often; and basically make the decision whether or not to strike a 'do I want to be weakened for a long time?' question.
If you go this route, concussion hits lost from 'super strike' shouldn't be heal-able via normal spell/herb means. You might allow some spells, like the ones that restore lost Co points to effect the hits... or create some spells/herbs. You could also use 'meditation' to double to rate of hit recovery during rest, or create a similar 'refocus life force' skill.
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Finally Damage...
Depending on how nasty you want to make the attack and the cost to the PC of preforming the attack you could go a number of ways.
-Add a bonus to the attack roll...
-Doubling the concussion hits and or crits..
-Giving the crit roll a +20 or more to ensure it's a nastier hit.
-Divide the crit roll by 2, then add 50. (forces the crit roll to be 51-100, but keeps 100's more unlikely then adding a number)
-My personal favorite is to make the crit a slaying, holy, or other 'type' of crit.
The neat thing about making it a slaying or holy crit, is that the PC now has a, albeit expensive, method of hurting certain, normally immune from damage, creatures.