First, you have to understand what
grasping roots does:
Grasping Roots now applies a non-resistable constriction effect to the
player (or target monster) when cast, with similar for escape for the
normal constriction used by e.g. naga monsters. The constriction damage
for Grasping Roots is higher than direct constriction, using two dice
instead of one. This is 2d9 for shambling mangroves at 13 HD, which is
high enough to be significant when applied over several turns.
So, the first thing to do is get out of constriction as soon as possible. There are many ways to do so, including moving, blinking, teleport.
After doing that, you can try to take care of Jorgrun, or escape. Jorgrun no longer casts shatter, so flying isn't necessary against him. You want to handle him like you handle any other powerful unique spellcaster. Silence, buffing yourself with haste/might/agility/berserk etc. are all good. If you have low MR, just escape, because you really don't want to be petrified by him.