Well... burst motor, iron spikes,an assload of red extenders?
Can't do it with pistons anyway, servos will probably fail as well so you've got either a hammer-like design or a clamp (which starts closed and is fired then clamps on the enemy).
Or you can have fun with changing a 90 DSL axle into a spin motor and give it very low speed and very high torque.
edit: ok wtf is with these ninjas
A burst motor and a servo piston might work, though it may need a special .py to work. Maybe have the burst go up and while that happens have the piston extend? The Burst will automatically then come down and since the bot is within the claw a smartzone could make the piston pull back.
That's practically begging the demons of havok and agod to come and possess your bot.