Kill oscar with a melee weapon while your wisp from wisp tea is on him. Make sure to get the final blow when the wisp is on oscar, my attempt just before this had my wisp leave oscar just BARELY prior to the kill and did not count. So try to be careful with your melee attacks on oscar for only when the wisp is on him. You can damage him with other weapons to get his health low before you go for the kill.
The trial portal of 5 waves with no perks spawns in the library and the relic is perk machines cost never decreases. Reference images here: https://imgur.com/a/chrtABW
You can take perma perks into the trial so get yourself geared up beforehand. Last wave is an HVT Oscar.
EDIT: To clarify, the video above is me activating the trial portal by killing Oscar. View the imgur link to see the portal location and the trial complete screenshots.