At the start of the Damage Step, if your "U.A." monster battles an opponent's monster: You can banish that opponent's monster until your opponent's 2nd End Phase. You can only use this effect of "U.A. Penalty Box" once per turn. You can banish this card from your Graveyard; add 1 "U.A." Spell Card from your Deck to your hand.
Well,this card works by banishing the monster your monster yse to attack and after you pass 2 tirns,you return the banished monster back to your opponent.That means by that time,your opponent would have summoned nonsters to replace the banished monster and when you return them the banished monster, since the card slot will be filled with monsters,it gets auto destroyed.
Sea Stealth Attack's effect will activate before Penalty Box's regardless of the turn player since it's mandatory and Penalty Box is optional. So Penalty Box will resolve first, banishing the WATER monster, then Sea Stealth Attack will destroy the U.A. monster.
Use with grass looks greener to get a free search, there is no restriction to the graveyard effect so you can use it NOW. Not tommorow, not after brrakfast, NOW. You can even use Double cyclone or storm to get some more value too
It's way worse than onslaught, guys. Onslaught is searchable by princess making it consistent, can banish multiple per turn, and permanently, special summons from the hand with boost, and special summons on destruction. Not even close.