If an attacking creature loses haste, perhaps because Fervor leaves the battlefield after attackers have been declared, it won't be removed from combat
The Onion Knight from the remake could also count as it potentially has the highest stats in the game and its exclusive onion equipment makes them practically invincible
Therefore a copy of such a creature will acquire the copiable values of the creature as it exists face down - typically a 2/2 colorless, costless, nameless, subtypeless creature - but it will be face up
Collector Numbering: Standard cards are numbered within their set (e.g., 1/204)
I will talk about all this news on this weeks episode of the PokeBeach podcast, along with additional insight.
To mitigate the risks of mana flood, one thing you can do is look for mana sinks