How to effectively train GYM in Pokemon Go

Even nowadays, many Pokemon players still doesn't know how to train gym. I see people using their strongest monster to beat everyone in the gym during training and ends up with 200~300 defensive points up. This is quite stupid since the game is not designed in that hard way.

Here is the trick:
Always choose the monster with a lower CP of the last monster in the gym to train it.
Formula:
CP of your Pokemon <= The last defending Pokemon  500+ defensive points gain
CP of your Pokemon <= 50% of The last defending Pokemon 1000+ defensive points gain.

There is obvious some rule in the types of attack type and Pokemon type. Please refer to the following chart:


source: http://www.pokemongodb.net/2016/04/pokemon-go-types.html

And keep in mind, there are some facts and tricks:
1. Super effective = 1.25x damage
2. STAB which stands for same type attack bounds = 1.25 x damage. For example, a Wigglytuff (Normal/Fairy Type) with a normal attack skill of Pund which is also normal, gives 1.25x damage than indicated in the discription
3. This is illy indicated anywhere in the internet, but if a Pokemon has a duo-type and each of the type is weak to defend a certain attack type, then it amplifies. So a Gyarados wish a Water/Flying type should be trained by a Pokemon with electric normal/charged attack, and this will lead to 1.5x damage. Most likely the Pokemon itself is also an Electric type, and therefore you make a ~1.75x damage in the end. This is a numerous increase of the attack and helps you to win a high CP Pokemon with a low CP Pokemon.
4. If you are a lazy baster like me, who doesn't want to spend time and effort memoriesing the whole thing, there is a site for you, http://pokewin.win/ to tell you to fight each Pokemon, you should best using which Pokemon with which kinds of attack types.

And the last comes two life-hacks:
1. If you can't find a proper Pokemon with low CP to win the training, just use Wigglytuff with Pound+Hyper Beam, it will ALMOST win all fights.
2. Put a Magikarp in the gym, and train with a grass type Pokemon less than 50% CP, and you will get 1000 + defensive bonus each time. Please bear in mind it's always an advantage to play together in a group, then the multiply effect is enormous!