I'm assuming all Sentinel.
Are you switching between the Eddies when playing? If I remember correctly, that will also cancel out the mission. I realize you may need to do that to keep them alive, but if you can, try it with just one of the Eddies so you can't switch between them. Maybe slap some of the enduring talismans (from arena store) on him so he doesn't die. Pharaoh also has that invincibility to your whole team when you use his Fury - so maybe stack those teammates with Thief talismans so your fury meter fills up quickly.
Strange little alternate option - I believe if one of your teammates dies, you can wait until they all die... then pay the 10 ironite to revive them, beat LOL with them all alive and it should count. Unless that bug was fixed.
This is what not to do if your bird shits on you.
That will work. And as far as I remember you can use more than one Eddie.
What I try to do is clear X with all my characters alive the first time. That way you'll get credit for both at the same time.
Some missions are tricky if you don't have enough good toons, don't be afraid to spend a few ironite to revive your team to complete the round, -you can do that. The rewards are pretty good, so you'll make it back in no time.
Ready solved. Just can use your own troops or toons. You can't use a friends troops