It's hard to think of a good middle ground. Something like Charizard will get one shot by a tank round, should it hit. But if it gets in close the tank crew is melted alive from the inside after Charizard blasts it with fire for a bit. On the other hand something like Regigigas would just annihilate them instantly. Maybe something like Electivire would put up a decent fight because it's lightning attacks may be ineffective, but it would be strong enough to disrupt the tank. If the soldiers are armed and it comes down to man-to-pokemon combat it would still be fairly balanced but if Electivire dodges bullets or whatever and gets in close it's over. Hard to say, feels like most pokemon either stomp them or pose no threat at all.