My preferred method for attacking a Turtled Opponent is what I call the 'Can Opener Roll'. Can't remember any of the real names for it. Start by standing to the 'side' of your opponent, assuming this is a Gi movement.
(1.)Grab the Gi around the back of their Collar while standing over them(bend at the hips to keep your feet out of immediate reach for them to get ahold of), fingers inside. Grab their belt with your other hand, fingers inside, so you could technically pick them up like a piece of furniture.
(2.)Go ahead and do so. Lift them up a bit or shake them so you loosen the defenses a few inches and...
(3.)...as you drop them down, step THROUGH the loop or arch made by your arms with the foot closest to THEIR feet and hook your heel into any gap you can find between their arm and leg.
(4.)Roll forward, over your opponent, into a somersault while your hook is dug in, and maintain your grip. Pull everything tight with you as you roll. Your momentum(if your weight is similar... This won't work if they're Sumo-class and you're not.) and strength will hike them up off the ground, freak them out, open their guard, and throw them over your roll.
Depending on your speed, their reactions, and some other factors, you should land in a near perfect position to quickly transition to a traditional armbar or Triangle choke.