Note: Wrote just for fun, and can be set just about anywhere in the series.
Merlin fell to the floor, tired and weak at finally vanquishing one of his greatest enemies. It had proven resilient to magic and nearly everything else he tried to use upon it, for its filth acted as a natural shield which repelled his many attempts.
But he had found its one weakness, something so very simple. He had a vague recollection of Gaius mentioning it to him a while ago, but it had been during one of Camelot's disasters of the week, as Merlin so mocked in naming them.
A simple, easily-labeled vial later, along with a long physical struggle, and his foe was defeated.
The leech tank was clean!
Gaius, consider this your birthday present!