Who is the strongest S1/2 rookie?

[Wisemon]

Now, ignore that stupid Pokemon rule, "Electricity doesn't affect Ground" when it affects any creature. Armadillomon's armour might not conduct it, but his skin would. His tail and ears are unarmoured, so all Tentomon would have to do is aim for those.

You think the Pokemon writers pulled that rule out of their asses? If you're grounded, you're very unlikely to be electrocuted; that's a fact. Tentomon would run out of electricity before Armadillomon got tired of being curled. Curling into a ball doesn't take that much effort. Try it and see for yourself. As far as aiming for Armadillomon's ears and tail, there's no way. Tentomon's stream is zig-zagged (like lightning), he can't be that precise.