9-cost Kaido has to trash one of your cards to activate his effect but will also force opponents to trash a card from their hand, bleeding them out of resources
These were featured in the Time Spiral block
The Rocky Helmet should also be used with Dialga to get a bit more damage out of attack trades, weakening your opponent's Pokemon
For example, if an effect has given Bear Cub, a 2/2 creature, -4/-0 so that it's a -2/2 creature, doubling its power and toughness gives it -2/+2, and it becomes a -4/4 creature
If you're just keeping it casual, "outside the game" means your collection
It allows you to put 10 damage counters on any of your opponent's Pokemon in any way you want