Use the Necro/Mesmer build in this thread:
http://guildwarsguru.com/forum/showthread.php?t=3048009
You shouldn't need it, but you can change out Energy Tap for Ether Feast. I find this build kills Incetol, Flower Spiritgarden, and Byzzr Wingmender very easily.
Apply skills in this order:
Spirit Shackles
Spiteful Spirit
Price of Failure
Reckless Haste
Be careful to stand on the wooden bridge just close enough to cast your chain without causing all the warden mob behind Flower Spiritgarden to drop. Then make sure you're either low enough on the bridge or behind the rock at the top of the bridge so that Flower can't wand you to death.
Kills him in about 1-2 minutes easily. All credit goes to Rebirther.
EDIT: Look here:
http://guildwarsguru.com/forum/showthread.php?t=3048440
for how to position yourself. Just be aware, that I don't use the build listed here anymore. While it can kill Flower, it's much slower than the one given in the first link.