He meant Ice Beam Serene Grace Blissey versus Mirror Coat Tentacruel.Tentacruel also doesn't have Serene Grace. The best option I can think of is still Serene Grace Blissey vs. Wobbuffet.
Here's what you do. lvl 50ish wob against a lvl 1 newly hatched (generic non-dark icebeam learner). The reflected damage wont be much. You should get like 5-6 reflected beams out of each baby.Test it by giving a Wobafet a lum berry and spamming icebeam vs him. Ingame. Right?
Even in the same turn, it shouldn't matter much. I'm sticking with this conclusion.In the first turn, the Tentacruel kept frozen, as I didn't give him a Lum Berry. However, in the following turn, he thawed out... and the Mirror Coat failed. So, it's not a Shoddy glitch.