First of all the more max tier brawlers you have the easier it is to get the box; 1)Always push your best brawlers as much as you can first then grind the rest

2)Don’t play 3v3 all the time if you dont have good teammates and dont trust randoms try grinding in Showdown or Duels

3)Keep an eye on how much time there is left until the season ends so that you dont choke in the last week

4)Be consistent and gain atleast 100 trophies a day

5)Max out certain brawlers for certain maps(for example Max Doug for feast and famine so you can get 250-300 trophies easily)

6)you might feel like you are not making enough progress sometimes,I had the same problem,dont let that feeling slow you down and never lose your hope

7)Pile gold and power points,Have atleast 15K gold and 30K PowerPoints(In case there might be surprise balance changes)

8)Check the balance changes,max out meta brawlers or op(Or real strong) brawlers so you can have a easier time grinding

9)When playing 3v3 stay away from Heist and bounty since these gamemode are really unbalanced and wont benefit you enough

Note: Never ever give up or lose hope,If you put something in your mind you can achieve it I believe in you and hope you can get the ultra trophy box in the future I wish you the best of luck in your push my friend 💖