Well, I'd need some cannon fodder gobbos, a troll, 2 giant eagles, a squad of lizardmen, a magma man and a gremlin to pull this one off.
Firstly, there are three entrances to the upper (old) fortress, one over the artificial pond in a canyon (which is protected by traps, and a drawbridge-gate over the pond), one through the volcano and under the magma lake and finally an old unused access shaft. The volcano and the shaft can only be accessed by a flyer. I'll use the first giant eagle to carry the gremlin through the shaft, ending up near the old trading area. Now, the eagle will stay in the upper levels of the old fort to act as a bait for the military, while the gremlin sneaks the mostly abandoned corridors to get to the lever controlling the drawbridge, allowing the rest of the force to gain access inside. The gobbos will go first, jamming the traps so the rest can move on unharmed.
Now, the military has probably already killed the giant eagle and, assuming the rest of the troops made it across the bridge, are retreating towards the lower fortress entryway. Now the troll must reach the windmill platform in the volcano that powers my main magma pumping array before the military reaches sanctuary and the "Burn Fucking Everything" lever is pulled.
Now that I must rely on my secondary pumps, the lizardmen can make it to the old fort's well (bypassing the fortifications blocking the brook side inflow of the aqueduct) and swim through to my main power plant. From there they must quickly head straight down to the sewers before the military catches them, and hole up in one of the waterfall receiver chambers so the marksdwarves can't shoot at them but they still intimidate any poor folk trying to get some sleep in the sewer.
The waterfall receiver chambers are next to the magma flood gates leading to the sewer, so I'd probably use the secondary magma pumps to obliterate the pests instead of risking a legendary miner to dig a new tunnel to them.
During all this the goblins have moved on to jam the weapon traps next to the drawbridge (now raised) that leads over the lower fort's magma river. Since the access of the magma river is disconnected from the rest of the river by fortifications, the magma man can't just swim through. However, since the sewer magma flood system is engaged to kill the lizardmen, the magma is now flowing. This allows the magma man to push through the fortifications, gaining access to the forges. From there he must quicly get to the nearby pumping station controls and pull the "DO NOT PULL" -lever that causes a minor magma flood to the area. this will melt the main drawbridge, allowing the second giant eagle once again to get the gremlin accross (This has to be fast, though or the reserve bridge is activated and the access blocked) Now the gremlin will simply sneak to the main control room and pull the lever that seals it off. No more fort.