The transformation doesn't necessarily make the card more powerful, but it maintains Flash, adds Double Strike, meaning you double the damage output in combat, and shifts the card draw from opponents drawing two cards per turn to combat damage dealt to players
In fact, it rewarded us with a promo pack every time we beat the bot on that level
You gain life equal to the damage dealt, but not more life than the players life total before the damage was dealt, the planeswalkers loyalty before the damage was dealt, or the creatures toughness
Now that you understand the themes and flavor of each color, you hopefully have some idea about the one you want to try first
The most impactful card is probably Exterminio, which would translate literally to Extermination but probably references to Overkill, the mechanic from Final Fantasy X, . Exterminio is a black instant removal spell with an interesting twist: setting a creature's health to -9999 plays around indestructible