Once you unlock Neville's waiter variant, you must beat one of the Bellatrix duels with him.
There are several duels with Bellatrix throughout the game however probably the easiest and quickest one is on the level "A Veiled Threat" just select Neville at the start of the level in free play, get to the bit where you have to duel with Bellatrix or Malfoy, select Bellatrix beat her and the achievement is yours.
To find Neville go to the courtyard outside the divination and astronomy towers Put on the Spectrespecs and walk to the middle of the courtyard. Here you will find some Spectre bricks which you can build to add onto the plant. When you have done this, the plant will open and give off a stream of air. Jump into that stream and you will be carried up to a gold brick and the token.