So ya, surviving 40+ can be tricky something. You really need to learn each mob type pretty well to know when you can be close to them and when you should kite off screen. For example if you get an elite pack with jailer you should be off screen 75% of the time, the only time you can move in is when you see jails cast on your pets/follower so you can head over and grab some health globes for Hatred regen. Either way, most damage is avoidable and timing is cruical with certain things like reflect damage, jail and thunderstorm. Once you can learn to avoid most enemy projectiles and time certain traits your rank will climb
Bosses are another story, generally they can take a long time. Try to save Pylons for bosses if they're near-by and accessible of where the boss spawns, sometimes kite off screen a bit to search for one if you know you're not going to make the timer. I've finished multiple GRs to make progression finding a Power Pylon 100 yards away from the boss I can't kill on time Either way, they're a pain in the ass as generally they're going to take you 1.5 mins to kill. There's a few you can stand off screen, plant your Sentries and just faceroll without having to avoid damage.
Saying all of that, it boils down to luck a lot of the time. Sometimes there are those impossibly annoying creatures with equally annoying affixes that will just *$(£ you up. You just gotta keep at it and hope for that almighty condensed zombie rift with no reflect damage so you can just pewpew your way onto the leaderboards!
GL with it man! I'm speaking from 44+ exp on my DH btw, nothing too shiny but I've got enough exp to help hopefully.
My DH if you wanna check her out.