I believe there are 2 reasons as to why Merlin was not able to defeat this adversary; reasons that are linked with one another, even though they can be considered separately. These are:

Trickler uses a different kind of magic to perform the love potion, one that Merlin is not used to.-Trickler’s magic resembles the old Irish magic used by the Sidhe, the fae. He says something that sounds like Lugh’s name when lighting the candle when he begins to prepare the love potion. Lugh is the Irish equivalent of Loki. He is a trickster deity, skilled in many arts, and has an association with fire. This is an important fact to remember when considering Trickler was lighting a candle-he is essentially invoking Lugh’s name and power to carry out a ritual that He has a direct connection to; a candle is a source of light that is created by a small light. That would explain why Merlin’s original spell didn’t work; his spell was simply not compatible with the one Trickler used because it was a distinctly different kind of magic than his. In addition, fae magic is far more ancient and therefore stronger than the standard warlock’s magic because the fae have been around for an extremely long time – it is the magic embedded in nature. It can’t be bypassed by your standard spell because it is far more advanced than the basic magic all sorcerers use. Trickler’s character portrayal as Lugh or Loki is further affirmed by his role as an unassuming jester at court (a jester is a textbook part of a trickster’s personality) and several lines alluding to Norse mythology in the episode.

“… My head, it is my prettiest feature.” (Loki’s thoughts to potentially losing his head at the hands of the dwarves after the bet.)

“And if I choose this one and it’s wrong, Vivian will lose all her hair.” (Vivian is Sif -Sif loses all her hair because of Loki’s jealousy.)

“You know, another meaning of the word jester is” (whispers) “keeper of secrets.” ( Using another one of Loki’s kennings.)