The Hydra's strength was slightly suppressed by the lightning blanketing and coursing through its body. But a mammoth without fur is still an elephant. It's tail easily broke apart every piece of Mandra's array it touched.
However, Mandra's spell pulled away the lightning components in the specified area and set a target. Those components were already gone, and the Hydra had already become the target.
The rest, the lightning bolts falling from the sky like divine punishment, was all nature's doing. Just destroying the spell wouldn't stop the thunderstorm.
But it did weaken the concentration, and the lightning strikes spread out. Several of them missed the Hydra. That still meant many of them hit the overgrown lizard, though.