Let me start by saying, I’ve never felt the need to go past UB1/2 in the past with how slow the grind was and how little difference it made in hourglasses.  The emblem was cool, but that wasn’t enough for me to play hundreds of matches.  
  
I started at GB4, but quickly lost my first 2 matches and dropped to GB3.  
  
I switched to my favorite deck to run, Guzzlord.  I sailed all the way to the end of UB2, riding multiple win streaks of 9/6/9 before hitting a wall.  
  
The basic strat of the deck is stall with Celesteela, load up energy on Nihilego, then ultra thrust the Space Jellyfish into the active and hit for 50, 70 with another Nihilego, and an additional 20-30 if Celesteela was barbed and the opp attacked the last turn.  I always dropped a helmet or barb on Celesteela as he was taking most of the dmg, and having a 2nd Celesteela really stalls out the opponent.  Never attach an energy on Guzzlord until Nihilego is up and running.  If you are able to squeeze out 1 energy on Guzzy before the Nihilego gets KO’d, you can Lusamine the 2 energies > attach one > hit for 120. The poison dmg should be enough to wear down opponents into KO range, or force a retreat where Cyrus comes in clutch.  
  
Grindcore won me many games, esp vs Charizard.  Team Rocket Grunt also has won me many games in the same fashion, and it’s a great option to run them back to back in a must win move.  
  
My fav strat is to deal and absorb as much dmg with Guzzlord, place my 2nd Guzzlord on the bench, and attach one energy.  Then on the next turn, retreat active Guzzlord (discarding 4 energies with it) > place the 2nd Guzz in the active > Lusamine > and (usually) win the match.
  
A few consecutive losses led me to switching to the Silvally/Oricorio deck.  I usually run an electric deck favoring Tapu Koko, but the flexibility with Zeraora with Gio really makes all the difference.  I rode this deck from UB2 ➡️ MB
  
Running 2 Gio’s, I was able to clinch a lot of early victories.  Going first was a luxury, being able to hit for 60 on your 2nd turn, OHKO’ing multiple basic mons (Eevee / Litten / Popplio / etc).   
  
Moreso, Gio was mainly the reason I won so many mirror matchups.  Gio is enough to hit the dmg threshold of Silvally from your own Silvally.  
  
Dawn, although not always used, was great when leading with Zeraora.  If Z-Cat has 2 energies and your Type Null is on the bench w/o energy, a simple retreat > Evo > Dawn would surprise a lot of my opponents with 100 dmg basically out of the blue.
  
If I didn’t like the hand I was dealt or the matchup, I would lead with Zeraora and load the bench with energy as Z-Cat was automatically getting one through his ability.  Doing this led me to quickly retreat and attack with a full HP Zeraora, Type: Null, or in rare cases Oricorio (Looking at you Incineroar decks).  
  
Going 2nd was just as good, given you had a Type Null to lead with.  Even without one, the Zeraora strat from going 1st still applies by loading the Null on the bench with Zeraora having essentially a free retreat.  
  
I switched from Mars to Cape during the last 8 games to help with certain dmg thresholds, not sure if 100% better, but it got me over the hump.  Ready to retire to random for the season and run new and fun decks bc I can’t bare to see these decks for a minute.  
  
FWIW, I never ran Darktina or FrogTina, although they probably would’ve saved me a few losses.

From GB3 ➡️ MB < 150 games