Put X +1/+1 counters on it, where X is the number of growth counters on this enchantment
Whichever logic you apply, going by that list will likely be the most sensible thing to do, simply because of its official origin
The number one question raised time and time again whenever the subject of MTG proxies comes up is: what does Magic: The Gathering creator Wizards of the Coast think of proxying
Thundermane Dragon's last ability allows you to cast the top card of your library if the resulting spell is a creature spell with power 4 or greater, it's your main phase, and the stack is empty
On the draw, you will have to rely on Thoughtseize and early removal to disrupt them long enough to take over the game
That being said, Nihil Spellbomb is better in the Lurrus mirrors and when Leyline of Sanctity isnt popular you get the graveyard hate and a card back instead of one or the other