The reason that boss is harder than the others, is because of the minion on the right:
Support #2 Attack 1 target champion and another random champion. Ignores Defense. Always land as normal hit. Damage Multiplier: 0.6 Target MAX HP
Since it ignores defense, and since it hits twice, it'll essentially always 1-shot UDK. The reason why Quinn is able to clear that boss so easily, is because he'll literally oneshot that minion, so it'll never do any damage.
For you, you have two options. First option is, like in her case, to take that minion out before it can attack. The boss will revive it, but as long as you can keep clearing it before either it or the boss gets a turn, you'll be fine, since the other one doesn't do nearly as much damage.
The second option is to CC it. None of your listed champs have any CC, though, so this option is probably moot.
However - the bigger consideration here is just whether you should even be bothering. One of the bigger mistakes newer players make is spending too many of their resources on leveling champs just for faction wars. Yes, it's a good idea to level *good* champs for that, but leveling random uncommons or rares just to try to progress is almost always a mistake.
Personally, if I were you, I wouldn't invest in any one of those champs listed, except for UDK and possibly Dark Elhain. Bring them to 60 - though definitely not as a high-priority task. There are almost certainly more important things for you to focus your resources on.