How do I add the increased damage effect on a mob?

If you want to increase the amount of damage the mob deals, then have your effect constantly give the mob strength.

If you want to increase the amount of damage the mob receives, then it's a bit more complicated.

Create a procedure with global trigger "entity damaged."

Global trigger: entity damaged
If ((event/target entity) has effect <your effect>)
Then deal <whatever number you want> damage