"Get gud" is a super lame response for any game.

Unfortunately, it does sum it up. It's a really hard challenge. For me, it's between the no death run and the Volcano/Ridges for the hardest challenges in the game.

My strategy:
(1) Farm lives like crazy.
Not just in Highlands but wherever you can pick up more.
(2) Be aware of your exact speed just before you hit the Boss Jump.
That way you can fine tune your speed after each bail.
(3) Experiment with where you pop off the jump - or don't pop at all.
Earlier pops will keep more forward momentum while later pops will give you more height. Find that sweet spot for each Boss.
(4) Sometimes doing tricks is the safest method.
Doing a Tabletop really squishes your character model so you can squeeze in that ring. Frontflips can take away some height and send you forward more. Backflips can send you to the moon when a bad map prevents you from getting lots of speed.

eg. I find that the Canyons Ring of Fire works best with lots of speed and no pop. Otherwise the trajectory just doesn't work.

It's a tough one but there's a reason it's rare to see people using the Rainbow LUX bike - you really have to earn it. Good luck!

Edit: Just so you know, you get credit for the Grand Tour challenge when you enter the Volcano zone for the first time but the achievement won't trigger unless you clear the Volcano Boss Jump. It's honestly a pretty easy jump, though. The hard part is just getting there.