First of all, if you're using Tallia and Demytha in your arena team, you need to rethink your arena team. We in fact need to stop and think about what a "pretty solid arena team" is. Cause that ain't it.
UDK is not unbeatable in the slightest, and shouldn't be giving you any problems at all with a Saltus to nuke him. Much less, you already have the direct counter you are asking for in Ronda. Ronda shuts down all his healing, and if you cant keep her alive long enough to do that, it's yet another reason you need to rethink your arena team.
Secondly, if you're only "working with the champions you've got" then you need to stop playing arena for a couple days, long enough to go farm a Warmaiden from Campaign, and then copies for all her books, so you can use an AoE 60% Decrease Defense champion and actually help your nuker push damage through.
One of your major issues is that you are ignoring a major arena debuff and expecting to win against anyone who is actually building champs properly. Let me break it down why you're failing your Staltus and Ronda here. Defense mitigates damage. Enough defense, such as what a properly built Ultimate Deathknight can have, mitigates up to 90% of your damage. This is why Decrease Defense is important, cause with 60% of their Defensive Stat gone, tanky and well built champs cannot mitigate such huge chunks of your damage. Then you can actually kill them, which is sweet! If you're struggling against lowly UDK, a well built Siphi, Duchess or Wythir is gonna really really upset you. Wait until you're facing enemies with 120k health that heal their entire team to full every turn.
Thirdly, if a Staltus cannot kill the lowly UDK, he is not "properly equipped" ... or perhaps he is not fully leveled. The only reason anyone should be having problems outputting enough damage to kill a UDK is if their starter champion is not finished being leveled, ascended and booked. And if they're still having trouble after that, they need to go farm some better gear in Dragon to get closer to 8k defense/attack, 100% Crit rate and 300% Crit Damage. And if that seems like a lot to you, or impossible, then you have a long way to go still on gear farming before a champ is "properly equipped" or anywhere near it. I would assert that just approaching 3k attack and 100% Crit Damage on my Athel was enough to take me straight into Gold 1... it was the entire point of this thread, and I included the screens to show this, and my terrible builds on that new account just to demonstrate how bad your builds could be and you could still be in Gold Arena.
Basically if you can't hit those gearing numbers, after taking a break to farm and build your AoE Dec Defense champ (Warmaiden is freely available), then it's time to take another arena break to go farm the gear needed to build your nuker better. Dragon is your friend. It's easy to get to 20, has solid gear you'll use forever, and will help you get build your Staltus to a point where you might start thinking "Wow, I might not even need this AoE Dec Def champ anymore!" (Hint, you still will).
I'll put it this way. Start your own thread. Include your roster and pictures of each of your arena champ builds. I don't want to hear you tell me that your champs are built well or whatever else, I want to see it. If my buddy @Balltazer can smack around a full Stoneskin, Ascended UDK with an Uncommon and a Soulbond Bowyer.... then hopefully you would recognize that your best option to make progress, instead of just complaining about how a champ is too hard or unbeatable, is to ask the people who can beat him with uncommons how they did it, and perhaps learn what it is they are doing to trivialize this guy. Then you can make the improvement necessary yourself! Improvement should be the goal, rather than asking the entire game to change because you can't currently beat a champ using your current methods.
Lastly, don't use language on my forums, or call anyone else's advice bullshit as they are trying to help you. Thank you.