I've been wondering this for a while and has recently been brought up by lansid_drakken in the gw.wiki section of this forum. I was thinking of creating a list of weakness here.
Afflicted - None i think -_- (so use sundering) - all elemental dmg is the same
Wardens - same as above
Giants - Fire
Undead - Fire/Holy
Ice Golem - Fire
Plants - Fire
Gargoyles - Cold
Devourers - Cold
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I always thought they'd be weak to fire since there flesh and fur.
All of their elementals are fire ele's, the brought the searing to ascalon, and they worship the titans.
) are VERY vulnerable to lightning. Did 195 dmg on one with Gaze from Beyond.