Given the power creep of attacks, HP and abilities in the decade since this Reshiram card was introduced, its definitely not a Pokmon that feels particularly competitive any more
The following rulings focus on the phasing keyword While a permanent is phased out, it's treated as though it doesn't exist
Whenever you cast an instant or sorcery spell or activate an ability, if that spell's mana cost or that ability's activate cost contains , copy that spell or ability
Its going to take some work, but he goes into great detail on the strategy here
Can you see the clouds and sky being featured vividly in the imagery
That said, it is extremely powerful if you untap with it, you likely win