First, the target requirement is ‘target creature an opponent controls’. That means if you’re just needing to use Trickster as a beater against an empty board, you can flash it in and not have to target any of your creatures, because you are not your own opponent (not even if you play as badly as I do). Second, you’ll note the lack of the word ‘untapped’ in there! You can absolutely use Merfolk Trickster on a tapped creature. You’ll basically ignore the instruction to tap the creature (since it’s already tapped), but you WILL still rob it of all abilities- that can be very useful as a combat trick on something your opponent is attacking you with.
There are times when that ability is really straightforward- you can use it to remove Trample from a big attacker so you can chump block it; you can use it on a flier so it can’t soar over your creatures (or use it offensively to take Flying away so they can’t block you!); you can use it to remove Haste right before an attack to buy a little time. It also works on ALL abilities the target has as the trigger resolves; Trickster doesn’t care if you have Flying naturally or gained it from a Maximize Altitude, it’ll get taken away all the same. Keywords are easy, but when you start deleting more complicated abilities, things get more fun. Let’s look at Crackling Drake.
Removing the Flying is straightforward, and removing the ETB trigger doesn’t matter (even if you flash in Trickster in response to your opponent’s Drake entering, their trigger is still on the stack! Deleting the ability from Drake does nothing), but what about the second ability? That’s what we call a Characteristic-Defining Ability. In this case, the Characteristic being defined is Drake’s power. Without that ability, there’s nothing defining the power- so it’s just 0. You can use Trickster to utterly blow out CDAs this way- heck, if you use it on an Awakened Amalgam it’ll go down to being a 0/0 and just flat out die- Blue creature removal! The same trick also works on Multani, Yavimaya’s Avatar, though that isn’t a CDA. Multani’s ability doesn’t DEFINE his power and toughness, it just adds to it; but without that boost, he defaults to a 0/0 and dies anyway. This trick does not work on things like Hydras- their P/T is coming from the counters on them, and Trickster doesn’t do anything about those. Likewise, Trickster won’t affect P/T changes from one-shot effects that have already happened. It won’t remove the +3/+3 from a Giant Growth, but it will immediately turn off your opponent’s Benalish Marshal and remove that +1/+1 continuous buff.
Today’s Rules Tip was written by Trevor Nunez