After banging my head against it enough for 3 clears an a few partial boss farms at CP1 here's a few personal tips/tricks.
1)Flashbangs/Gemini Jesters are best friends, the Dungeon is *Add'oclock* all day ,so slow em down.
2)Slow an steady, don't rush this, clear out adds an come back , especially in the tomb/vault cp.
4)If you cause the Flag bearer to spawn they stay there. So don't worry, go back , reset the clock an come back.
3) DPS isn't king . There is no need to rush this.
4) Watch the clock, at 15 seconds , bail on whatever you're doing an go back.
5) Use whatever keeps you alive, screw the *the Meta*
That's it , just relax , reset an recenter.
You got this Guardian.
Drink your water an don't lick rocks