Remember that creatures have summoning sickness when they first enter the battlefield, meaning they cant attack until next turn (unless they the haste ability), though they can defend against incoming attacks (more on this later)
But villains don't play by the rules
This widens the options available to deck builders
Hydras are the perfect tribe to do this with, as their flexible casting costs means you can always adjust your newest creature to be stronger than the last one you cast, triggering Selvala and drawing you an extra card
Make sure to check the expiration date for each code, as they are usually valid for a limited time only
In a similar vein, Expert Judge X brings up an excellent advanced technique in this category: the dance of having the Spray. Here, upper-level SP players use several ploys to trick their opponents into thinking that they either do or do not have a Power Spray