As I see it, the biggest problem is the ultimate spams. Which, although it has proven to be an unpopular opinion, it would really be solved by nerfing the concentration potion. That would work like this: “A concentration potion, when drank, instantly refills your ultimate bar. The end.”
That would still be a good potion to have, for those instances where you think “oh man, if I only had my ultimate” and it could still be used to increase boss damage although in a less broken and more time consuming way than it currently is, IE: BH (or shade, or whatever) uses its powerful ultimate, then has to drink a potion to instantly use it again. unless potion duplication kicks in, it has to fight normally from there, which, it seems to me, it is the desired goal of the monster/lord discussion.
IMO this would be better than giving the boss thresholds or time-based damage reductions, which could easily prove unfair in certain circumstances.
After all, if a Bounty Hunter, a Shade and a Grail Knight w̶a̶l̶k̶ ̶i̶n̶t̶o̶ ̶a̶ ̶b̶a̶r̶ all focus on a rat ogre, I expect it to die, or get close to dying anyway. While it feels more unfair to the bestie if a single player stunlocks and deletes it in seconds.
While we are at it, I’d maybe change double shotted so that it does even more damage to a boss, but without the 80% cooldown, which, granted, it is skillbased, but it also looks pretty ridiculous.
TL:DR: I’ve highlighted the most important parts, but basically: I think nothing would be better for monster/lords balancing than changing the way the concentration potion works.
For people concerned that it’s too big a nerf I can only observe that between the 2-4 potion slots, potion dup option, and the sheer amount (and popularity) of boss-deleters I think there’s still a way to reach the current “laugh at skarrik” level of unfairness, if it’s something you absolutely feel you can’t do without. Just, not as ridiculously easily.
PS: this would make concotion useless on purple, but I think it’s a good thing. It could also be a reason to run potion duplication. Decanter might need to be balanced though.