Slaking/Boost

[Ancient Pokemon Trainer]

Question:What happens with Slaking's Critical Move with Boost

Lets say for example there are 4 (L) energy attached to Slaking.You attack with Critical move.Next turn you get Boost attach to Slaking Critical move you must discard 2 energy can you discard Boost?

[PokePop]

I'm not following what you're asking here.
You have 4 L energy attached.
You discard one, use Critical Move.
You can't attack next turn.
So, if you leave Slaking Active, you don't get to attack, Boost or not.
Now, let's say you get him to the bench and then bring him back up.
Now you can attack.
If you attach the Boost, you still have to discard an L energy because the attack requires you to discard a Basic Energy card, and Boost is a Special Energy card.
After your turn ends, you must discard the Boost, whether you attack or not.