I’ve been doing stuff in SL on my level 80 main and have been working on my covenant things (currently Kyrian) when I ran into this annoying achievement. “All The Colors of the Painbow” requires a path of ascension enemy to hit you 10 times in a row which just isn’t possible any more on anyone in the high 60s or above. Party sync only brings me down to 70 and they still can’t hit me. It’s even worse that the achievement’s still character-specific and you can’t purchase the mount until you complete it. I did some research and I guess I could grind a ton of anima and some quests to level up a completely new covenant or level Kyrain again on a new 60 but uuugh I wish this wasn’t the only way. Especially cause now I’m just missing some personal achievement points on my main 80 
I’m pretty sure you cant dodge while seated, make a macro with /sit and spam it is my advice
try removing all of your armour and maybe even your weapon.
Doesn’t even help at level 70.
Even with a macro, Pelagos cannot /sit during the battle.
I have been trying to get this achievement done on my level 70 Spriest. I have managed to see 1 stack of Unstable Stimulant hit, but it faded before more stacks could hit.
turning your back?
Nope. Didn’t help.
Its achievements like this that always worries me about this game, especially ones that are tied to meta ones. This achievement, another that comes to mind is getting Legendary on the Tuskarr Soup WQ which is not possible by oneself as far as I can tell/have attempted, or getting 10 people to hook onto a single fish.
Its frustrating, and I’m sorry you’re struggling on this. I can’t blame you for being frustrated and looking down the metaphorical barrel of having to level up an entirely different toon JUST for this one achievement even with the Adventure Mode addtion that skips the opening storyline.