Most unkillable comps are going to require high speeds on one or two champions. I think there are some where you can go as low as 240s but if you can't reach that yet you will need to go farm dragon and faction wars.

Demytha is used in a lot of great comps and I would suggest getting to the speeds needed so that you can top chest unm daily with 1-2 keys as that changes then game dramatically at that point.

Lore of steel mastery will help get a bit faster.
Faction guardians can help get a little faster.
Glyphs will help a lot but when doing these comps 1-2 speed can throw off tunes so it's good to save up a bunch of 1-3 star speed glyphs to get the numbers right.

There is no easy answer other than grind out the gear. You can post your roster and maybe someone can help with a team.

I have played around with a demytha warcaster UK build quite a bit recently so can will give some thoughts 

The first is that this type of build will be running 1:1 with CB.  A challenge with this is that your damage output can be quite low compared to a 2:1 build, but it is possible to still get a 3 key (or even 2 key) UMN build out of a 1:1.  You do really need to have some dedicated damage champs such as Geo or Frozen Banshee though.

Speed-wise, just use this calculator https://deadwoodjedi.info/cb/41792d552aa0d818bbb84fc1159f21e48522683f.  Essentially the speeds are

Demytha - 176

Warcaster - 248

DPS1 - 177

DPS2 - 186

DPS3 - 178

The only speeds that really matter are Demytha and Warcaster.  The rest can be anywhere between 170 odd and 188 odd.  The reason being that CD's speed is 190, so this builds a solid 1:1 team.

As you have identified, the challenge is the stun target.  Crimson helm won't be a good option as CB will prefer not to target a champion with block damage.  As a result, he will likely target someone else.

In my testing Torturehelm won't work either, because while his self revive is on a 3 turn CD, the issue is that when he jumps back up his turn meter is back to 0.  This means that on the turn after he revives he won't get a go, and then he only gets 2 turns before the next stun attack, which isn't enough to reset it.  

If you make him any faster than CB (e.g. 191) so that he can get a turn when he revives, then he will throw out the rest of the team and actually end up taking the AOE damages.  If there's a tune that works I cannot find it

Edit: I haven't tried it but Gnarlhorn has a 3 turn unkillable self buff that could potentially work, though you would need to clean the stun afterwards (or bring a block debuffs on a 3 turn CD too).  That said, he brings nothing else to the table at all, but he could be kept in very light gear to ensure he is always the target.

250ish speed on Warcaster I should be able to handle, the ridiculous speeds I'm referring to is rather the ones in the 300s or close to. 248/176 is the one I've been looking at, and my hopes was that I could find a tune that would make the stun and lack of protection for that turn irrelevant. I tried doing the math and couldn't find a way to make Torturehelm properly sync up, so I was just hoping I was missing something.

I was hoping to be able to get Demytha to effectively purge the stun with her A2, if I can find a tune that can accomplish that. It doesn't exactly solve my unkillable problem, but I suppose a Gnarlhorn might work there. It also causes a problem for the affinity with a speed debuff, and while I suppose I could sacrifice either Tatura or Mausoleum Mage to throw up a block debuffs, it's a bit much to use up 4 slots for what is effectively support. My hopes were 3 slots with Frozen Banshee and Geomancer doing the damage. I also have Doompriest, but I need her for DT so that would be painful to lose.

The above work, it is with warcaster at 4:3 and the other at 1:1.  I did this with Demytha, warcaster and tatura to block debuf for the stun and the speed decrease in spirit affinity.   He is also the stun target and he survive until the end (sometimes he dies at turn 48, but the team is good for the last 2 turn with block damage)

Lately I changed to have ninja also at 4:3.  To do more damage, and the speed requirement for Warcaster is a bit lower.  You can see the speed below

You can replace ninja with another dps but at a bit higher speed (I think it is 222 or 225)

I tried with Geomancer as a DPS, and the damage was around 12M.  Not as high as in unkillable team, with block damage he reflect 0, but there is a giant slayer proc.  I do not know if Fayne, Rhazin or other easy to have DPS it would be higher.

i

i

Now this a team I can build speeds are not so insane and have all leveled but WC.

TY for posting it.