What the achievement asks
Throw a Sporonite Bomb at The Lichen King, a rare in Stormsong Valley. Five points.
It is a nod to Ensidia's Lich King kill in 2010, which is where the name comes from. Mechanically it is two items and one piece of timing.
Getting a bomb
Sporonite Bombs come from truffles, which are invisible without a Pig Nose.
The first Pig Nose is free, inside the Carved Wooden Chest in Briarback Kraul, which is also one of the ten treasures of Stormsong Valley. Looting it unlocks the vendors, so you can buy more afterwards for 25 Briarback Quills each.
Sporonite Bombs can also be bought on the Auction House, which skips the whole truffle hunt. Worth checking before you spend an afternoon crawling under trees.
Buy more than one. They are not rare, and the mistake below costs you a bomb.
The timing, which is the whole achievement
You must be in combat with The Lichen King before the bomb lands. Thrown at him out of combat it does nothing, and there is no message to tell you why.
Pull him first, then throw. The achievement fires the moment the bomb hits, well before he dies, so you will know immediately whether it worked.
There are recent reports that merely engaging him is not always enough and that he has to be wounded first. If a clean pull did not work for you, hit him once with anything before the bomb.