After you've killed the other two elves, build some Dwarven towers just outside the range of the mightiest elf. To figure out the correct location, build one close to him, if he immediately destroys it, move out a bit until he no longer destroys the tower.
Note that the constantly spawning harpies tend to destroy pre-built towers, so after you know the correct distance from the elf, build 2 at a time. Then slowly expand the line of towers, building them as close to one another as possible. When new towers are right beside finished towers, the harpies aren't able to easily destroy prebuilt towers and buildings. You can also create a few dwarves and set a scout point at the towers. Dwarves will quickly build towers for you.
Next, build a temple of Crypta right outside the Dwarven Towers. I built Dwarven towers to the north and east, and the temple to the east, just behind the towers. You'll have to retire any Temple of Agrela (healers) in order to build a Temple of Crypta. This isn't a problem, as healers won't help kill that last elf.
Upgrade the Temple of Crypta to level 3 and then train 4 Necromancers. Necromancers are worthless against the elf until the temple has been upgraded to level three, so don't waste money training any until the temple is fully upgraded.
Cast the Wither spell on the elf every 30 seconds or so (this costs 300 gold). Then send all 4 level 3 Necromancers against the elf. Some will walk right into his range and be killed, but eventually, some will stay out of his range and send waves of undead against him. The undead will die, but the necro will stay alive. Whenever a necromancer is killed, quickly train another.
Eventually, the elf will move towards the necromancers where he'll run into the many Dwarven towers you've created. The dwarven towers arrows combined with attacks from the the undead, and the repeated Wither spells will eventually kill him. I've also found that regular warriors tend to last longest against the elf. I sent a few of those as well.
The main reason that last elf is so hard to kill is that he regains his health very quickly. If your reinforcements have to march all the way across the map, he'll be healthy be the time they arrive. This is why you need to build temples right next to his location- so when a Necromancer dies, another is quickly able to take up the fight. You may even want to build 2 temples of crypta just outside his range. Perhaps even a warrior guild there as well. If you don't have any dwarves yet, build their fortress near him as well.
Note that keeping up the wither spells and continually creating Necromancers and warriors can cost a lot of gold. If don't have enough gold to keep this up, build another market or two ( or three ) before starting this final push.