Bring enough booze and food to last through winter (about 60 plump helmets and 60 dwarven ales), give two of your dwarves marksmanship, a pair of copper crossbows, and 5-10 bolts each. Give all of your dwarves novice wrestling, and make one an axedwarf.
Bring one pick and one axe. Don't bring an anvil. Bring a few bits of wood.
Bring war dogs. Lots and lots of war dogs.
As soon as the game starts, assign your dogs evenly to all your dwarves, so that each has 3-5 dogs at least. Make one of them the leader of your military, assign the proper weapons, and send them into one of the towers. MAKE SURE it's not the one with the civ leader or the demon in it.
Once this tower is cleared, dig out a little farm and a mine under it, and start work as usual. Try to avoid sending your dwarves outside, and DO NOT reclaim items outside of the tower you've taken.
Occasionally, a guard will wander over to your fort. Try to keep your dwarves working in a centralized location so that they don't get ganked. Use the wood you've got or can safely cut to make some beds and charcoal, and start melting the reclaimed goblin weapons and armor as soon as you can. Set a refuse pile inside and get a nearby workshop to churn out bone crossbows and bolts (and maybe some armor) as fast as you can. Build a trade depot just outside your tower.
By the time any traders or migrants arrive, you should be pretty well armed, but don't go rushing off right away. What you want to happen is for the traders to wander through and cut down some goblins on their way over to you. The more random wanderers get culled, the easier time you'll have when it comes time to snuff the rooster.
To take the second tower, draft a fresh military, leaving enough dwarves behind (at least 7) so that you can recoup if you fail. Send an army of bone and iron-armed and armored fighters into the second tower--THE ONE THAT DOESN'T HAVE THE CIV LEADER. HE WILL EAT YOUR FACE.
Once you've killed off the second tower (and third, if there are four), you'll be free to move around a bit. You'll need a pretty hefty force (10+dwarves in iron plate mail or better), plenty of crossbowmen, and an army of dogs to even have a shot at the leader. Don't be afraid to leave him moping around in his tower for a year or more if that's what it takes.
Above all, be patient. Your seven dwarves rocked that first tower, but will not last long in the second.