What the achievement asks
Exalted with Marasmius, the mushroom under Ardenweald. Five points, account-wide, and the title Fun Guy or Fun Gal.
No difficulty at all, and no way to rush it. This is twenty-eight days of five-minute visits.
Getting to him
Research the Transport Network from Zayhad the Builder at the Heart of the Forest, then take the quest Silence in the Stalks. It leads to a cave under The Stalks, where Marasmius and his quartermaster live.
Rank 3 is the whole optimisation
Each rank of the Transport Network adds one daily quest. At rank 3 you get three, worth 350, 500 and 650, so 1,500 reputation a day.
Exalted is 42,000. That is twenty-eight days at full rate, and considerably longer at rank 1.
Push the Transport Network to rank 3 before you start counting days. Anima is easy to come by now and every day spent at a lower rank is a day added to the end.
Reputation bonuses stack on top, so a Darkmoon Faire buff or a racial reputation bonus genuinely shortens this.
You cannot see your progress
Marasmius does not appear in your reputation pane. Not at Friendly, not at Exalted, not ever.
The only way to see where you stand is to travel to his cave and open his dialogue window, which is a nuisance when the whole point is a quick daily visit.
Because he is not a normal faction, Exalted with Marasmius does not count toward the 100 Exalted Reputations achievement or any of its siblings.