Gyms

Since its an enemy Gym, why should i fight with half CP Pokémon?. I would pick my main attackers and take it down quicky.

Doesn't matter. Use your best attackers. It will lose 1000 prestige after each successful Pokémon defeat.

Like others have said. When attacking enemy gyms always use your strongest pokemon. You always knock off 1000 for each enemy pokemon you defeat and an extra 1000 for the last one (gym "boss").

When prestiging against your OWN team's gym that is when you use your 1/2 or "lesser" CP pokemon.

I'd use my 2800 vaporeon to take down the first two, piece of cake, and my 2400 jolteon to take out the vaporeon

and yes, I never put my offensive pokemon in gyms to defend. I have a whole other list of pokemon to defend.

You would never use smaller mons on rival gyms only training friendly gyms to get max prestige. Id use my 3k's on those a Gyrados b/hp to start and Dragonite DB/DC on the Vape.