Like most Pokmon cards, the value of Pikachu ex depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Prismatic Evolutions)
If a creature takes damage that isnt enough to destroy it in a single turn, that creature stays on the battlefield, and the damage wears off at the end of the turn
So how do you strike that balance
Devoted Crop-Mate: It's like Detain from Return to Ravnica, except you use it on your own creature for a bonus
For daybound/nightbound you need to start tracking the state of day and night when a card that cares about it enters the battlefield
Card advantage is a huge factor in every game of Magic, so being able to completely sculpt an opponent's hand is ridiculously strong.