All right... here's what I've learned from my experiences with Enders.
They are a great challenge!
They are NOT impossible to kill. Just very difficult.
Considering their abilities, no stun, no tire, no bleed, etc. I looked at the various weapons and what would be needed to kill one. First, let me say, the bezerk merchant left, allowing further Ender merchants to test my defenses. With my drawbridge and floodgates up they come up to the edge but don't get in. For some reason after that they spotted a door I had been using to contain my magma in a nice small area and decided that they wanted to destroy it. So they hopped down into the magma and obliterated my door. No biggie for me. It just led to the moat of my first trap chamber. Since I had a magma pocket and therefore a limited amount of magma I had wanted it contained. Still, no biggie. But note this:
Enders die in Magma
Every time they came afterwards they'd hop right on down into it and all kill themselves as soon as they'd arrive. I'm not completely sure why. I suspect the pathing route is quicker through the magma. But it still kills them. Perhaps removing the [Magmavision] tag or whatever would resolve this. I'm not completely sure. I'm on year 11 of my fortress and approaching the 80 dwarf limit needed for sieges and I'm all geared up and ready.
Still, I'm not sure they'll come, since the only civ's I'm aware of are at peace with me. Unfortunately. I can only hope that it'll happen anyways. Several of the ender merchants are flying at the edge of the map, melancholy or raving mad, but they won't leave. They're also 'friendly' so I can't attack. So for now, no more merchants until I figure out how to get them to leave or kill them. Maybe I'll try building a platform above them...
In any case, back to the weapon analysis. By my estimation it would take at least a well-crafted adamantine weapon to bypass their damblock of 50. In any case, every single dwarf in my fortress is trained in weaponsmithing and no other skills that might be used (where possible). Only two of my initial seven handle all crafting jobs. Due to this every artifact I've gotten so far has been an adamantine weapon. Now, since internal damage does nothing... crossbows and spears are absolutely useless. Maces and hammers only stand a chance of a kill on a critical to the upper torso, possibly the lower torso and head. Battle axes and swords are my preferred choice, but since swords have a critical boost and that's what's needed for actual kills all my guys are trained in the use of swords. Swords do less damage than axes but the important point is to get a critical that stands a chance to sever a body part. Enders without arms can't use them to attack. Or throw fireballs. (shudder)
Because of the importance of training and good weapons I've got 20 champions that are legendary in sword, shield and armor use. They've all got exceptional or masterwork adamantine swords. All have good steel chain/plate armor. Except my original five. They're legendary miners/engravers/pump-operators on top of the others and have been training with copper weapons for eight years. They made champion after about a year and a half of training. Due to the nature of my defenses, all goblins are funneled through a single staircase into the middle of a room where my soldiers will be stationed, minimizing the visibility and potential for fireballs. Additionally I've got cages of war dogs to be released to help provide fodder.
Granted, I still wanted to be sure this would work so I saved, abandoned the fort and started an adventurer, going to claim my artifact weapons and adamantine armor from my fort before attacking an ender goblin fort. What fun I had! Granted, fireballs suck, but other than that it was pretty reasonable. Firebreath it may be noted can be blocked with a shield. I did not train up my guy to uber stats, rather I just got him the best armor and weapons possible and went and had fun. Here are the results.
As you can tell, it's not so much the skill that's required, but the equipment. Therefore I'm confident that put in the hands of my elite soldiers the adamantine weapons and armor (enough armor to fully outfit the original 5 at least, plus whatever I can get *Below* which I've not done yet.)
will be sufficient to kill Enders in a siege setting. As an adventurer it was a difficult to kill one, a severe challenge to kill two and near suicide to take on three at once. (I managed, but only with severe injuries, spinal damage and losing an eye, after which I died trying to exit the map to Travel) Still, I soloed it as an adventurer so with 20 trained and equipped soldiers the numbers may balance out. I'll continue with my fort to see if I can get them to siege yet!