Nice, where is Trialmaster spawn ratio buff?
I've talked with the team about this a few times since Ultimatum launched. It takes roughly 100 Tier 11-13 maps to spawn the fight, or roughly 50 tier 14+ maps on average. We've tested this multiple times to make sure this rate is matching the number of encounters actually being done and it does match. Part of the issue is that some people aren't making it to the end of their Ultimatum trials to spawn him, some people aren't running enough high tier maps and some people who are meeting those requirements are contending with bad RNG. Relative to other league bosses, the Trialmaster has a more frequent spawn rate. I've seen some people speculate that there's a hidden condition required to spawn him (like win streaks) but this is not the case. Your best bet is to be doing trials to completion in the highest tier maps.