They can only be knocked back when they don't have the buff they gain from being with range of other elementals - or in other words if you are AoE'ing them as a group they can't be knocked back. We're still trying to figure out the best way for our comp to handle them as well, I think that either the stack in the middle then spread in a circle strat or the kite from one side to the other but then spread out strat will be what ends up working because I just don't think we'll get them down via AoE like Paragon's strat.