Jolteon is great as an attacker due to the abundance of water types and the fact that when I'm paying attention I dodge the oncoming attacks. But I'm not sold on defense. I've used him as one. But the more I see gym battles, the more I've come to see that the computer just toes the line and hits as hard and quick as it can. Which doesn't bode well for a Jolteon unless you've got a much higher level Jolteon than your attackers. But then I've yet to come across a gym that I can't take down. The best defense would be a gym with at least 5 or 6 defenders at a high level, with the Pokemon types being less important than the levels at the moment.
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Jolteon as a gym defender
Just seen jolteon listed as one of the best defenders of a gym in the game. But i dont see how as it has neither very high DEF or STAM/HP.... anyone have any ideas?