Did the casual event today, was annoying. Everytime we got to the turtle, he’d burrow away.
BUT!
I was on Paladin, something that helped me in terms of breaking his shell was throwing a Destruction Bomb when he becomes counterable.
If you suck at countering like I do, throw your bomb when he turns blue. Staggered and broke him 4 times purely by hitting him with the bomb instead of trying to land a counter.
Just thought I’d point out something that worked well for me.
Instead of whining and asking for a nerf, he learned and shared his experiences.
+1 respect
Interesting. I thought items and skills that could counter were marked “counter”. I’ve only been using destruction bombs on weak point attacks.
Corrosive bomb is a better choice for weak points.
Also, additional information for OP and everyone else stumbling onto this thread:
When a skill says “Weakpoint” as per below, it means it will inflict “Destruction” like a destruction bomb would. You can use these skills as an alternative to the bombs but it’ll take a bit longer to break. You can obviously also throw a bomb and then use those skills for further destruction.
(this skill by virtue of a Tripod)
Corrosive bomb does not deal destruction. Rather, it enhances / boosts the weakpoint effect.
As per the example of my skills which are Weakpoint +1, the Corrosive Bomb would enhance them to Weakpoint +2.
So, another viable method of breaking something is Corrosive Bomb → Weakpoint Skills and the remainder of the party throws destruction bombs.
Yea, I switched some skills around for weak point, so did the wife. But having to deal with the turtle and darkness, it was easier for me to throw a bomb because I was always out of position for a counter, purely by dodging mechanis of the two guardians together
For sure, I sometimes just burn flares because I can’t see the boss but can see the minimap icon 
That darkness was the real MVP. Luckily I went in hybrid healz/stagger. 4 blue 4 yellow so I was able to cleanse the darkness with my bubble when it was up.
Still a pain though