When Merfolk Trickster enters the battlefield, tap target creature an opponent controls. It loses all abilities until end of turn.
4/27/2018 Once you announce that you’re casting a spell or activating an ability, no player may take other actions until the spell or ability has been paid for. Notably, players can’t try to remove a permanent’s activated abilities to stop them from happening. Once activated, the ability on the stack will resolve even if the creature loses the ability.
4/27/2018 If the target creature has an ability that triggers when it becomes tapped, that ability triggers before it loses all abilities.
4/27/2018 If the target creature has power and toughness written as */* with an ability that defines its power and toughness, it’s 0/0 when it loses all abilities. If its power and toughness are written as */*+1, it’s 0/1, and so on.