Colru probably is harder at first to be honest.
The main problem is that Colru has 4 phases, and you'll need to learn how to deal with each of them. This combined with the pressure of not dying during the miniboss and his unblockables means he probably is about 4-5 times harder to learn. The other problem is that certain forms (Gray and Green come to mind) completely murder half the classes.
Lionotus is only really hard for most people since it's hard to grasp that cliff dropping and knockdown spamming are more important then DPS. Lionotus is actually nice in that you can usually fully heal on the ramps without too much risk. That means as long as you can tank a hit or two (mainly his two hit combo for Lanns), you can run to the ramp and heal up again.
Well, for the unblockables in Colru miniboss, they're slip-dashable, timing is very simple. As for a Lann in Green, it's just a matter of slip-dashing when the fire aura appears. And in Grey, very simple tactics, just a chain of slip-dashes well placed and he'll be wide open to 2 DCs.
@above: Lionotus is actually very easy, the slip-dash timing isn't hard AT ALL, it's just the placement of the slipdash. The trick is to slip AWAY from his massive frame, or if he's attacking you, to slip INTO and THROUGH it. I would demonstrate it but haven't been playing at all as of late.
Accomplishments...
Well, it's about a week and a half back (forgot to update+is retiring) but I got 1st place in another glas, I remember it oh-so clearly because of all the deaths that occurred from people not knowing what to do.