Discussion on Level 10 gym

Impossible if the 10 Pokémon in the gym are ALL lapras.

Yep, just use your biggest and best attackers. You have a little over 90s for each pokemon in a gym. If you need help filling up your team:
- Exeggutor will resist Zen Headbutt, but be weak to lick.
- A Snorlax of your own will resist Lick, just be sure yours has Zen Headbutt.
- Rhydon will resist Body Slam and Hyper Beam, but be weak to Earthquake.
- Gyarados and Dragonite will resist Earthquake.

The CP adjustment is done only when training in a friendly gym, not when attacking an enemy gym

This is the 2nd main reason gyms aren't torn down daily...Possible to do, but low rewards for doing it (exp which is less than, say catching 1 mon giving stardust as well) and as you say, a waste of time (and potions) really.