If this robot is for a battlebots competition, the only two recommended ways are High pressure gas actuator or the way "Grendel" the battlebot did it. I tried looking on line for specs or some kind of mechanical diagram for Grendel's, but to no avail. Daniel Rupert's sites, the guy who built it, are all out of commission. However, here is a video of Grendel defeating one of my favorites. Grendel is the black one with the monster face on the spike.
The Grendel mechanism worked by a motor pulling a cable to tension a spring, after all the tension was achieved the clutch would disengage forcing a lock to secure the tension arm in place. When ready, One press of a switch and the lock would release, the spike would come down with all the force of the spring. This is cheaper to make but not quite as exciting as a HPG powered actuator action as in the case of Grant Imahara's DEAD-BLOW.http://www.deadblow.net/Pages/99weapons.htmhttp://www.deadblow.net/Pages/00WEAPONS.HTM
In either case, instead of a spike or hammer, but an ax at the end and you pretty much have your weps sys.
HOWEVER, if you are making a firewood robot to operate and chop wood in war environment... I need more details...