It does seem like they got a lot weaker, thought I guess it might just be the huge fuse variance they have. Two of my Juggernauts used to maul the Leviathan, but now they lose to the time limit.
Plasma fire also gets easily intercepted by the point beams...
I guess the most prevalent (and I would say, the most effective) strategy is to use 4 Enyos and keep blasting the Leviathan till it drops. I can finish it off in around 1:15 at my best.