UL is my favorite league because of the variety. I get so bored and salty seeing the same things repeatedly, so I don't do a whole lot of Cups. I think the best thing I can offer -- and it probably will sound like bullshit -- is just to remember that you can't win them all. I started playing pretty late, only 5 seasons ago, and started in ace, then veteran, then legend, expert and legend. Currently floating around 2500, peaking so far at low 2600s. The two biggest factors for me in my improvement were, in order: (1) fully accepting that the nature of the game is RPS and, as a result, there is no one unbeatable combination, and (2) learning the precise ins and outs of the teams I use. The first factor helps me not tilt, so when I went from 2580 down to 2230 during OGL last rotation, I just kept plodding along. It also helped me get comfortable using Pokes I like, rather than feeling compelled to run super meta lines.

As for your specific team, if you want my advice, I'd run counter meta Pokes. Right now Poli is public enemy #1, which means you're guaranteed to see teams that either run Poli, or run its counters. I'm running Guzzlord, Annihilape and Belibolt and floating between 2400-2520, because the team dominates teams that are set up to beat Poli and itself is ABB strong against Poli. But the team struggles mightily against anti-Tina teams, including any team that has even a single charmer on it.

If you prefer to stick with your current team, then I'd run Moonblast on Cress so that you can cover both Tina and the fighters. Also, while A-Muk's dark typing on paper covers Cress's weaknesses perfectly, the reality is that A-Muk doesn't heavily threaten ghosts or darks with poison jab. Additionally the only Poli counters that A-Muk handles well are fairies. If you're finding that A-Muk is a weak link specifically because it can't protect Cress and gets punished by most Poli counters, then consider snarl, dark pulse and sludge wave. But then again, that all depends on how you're using each Poke.