im not sure what mesmer spell you are using but make sure you dont take any energy taps or heals. This will only make your mirror harder to kill. The best bet at least that Ive seen is to go into the fight with heavy damage. Dont worry about glyphs wards or anything as again these make the mirror harder. Hope this helps
Put up like 5 or 6 useless spells and then the rest nukes. Blast away while the Mirror casts other junk. I have a E/Mo and used things like Ward vs Melee and such then just unloaded on him. Dead in about 25 seconds.
As lame as it sounds... the best and only way is to go in fierce... attacking with the fastest high damage skills you can. Don't try anything fancy (draining skills), just wound, wound, KILL.
Took me two tries to kill my mirror. The first time I tried to use my skills... the second time I pummeled him with my fastest, most damaging skills and he dropped in 6 or 7 hits.
Lightning Orb
Chain Lightning (Why? i dont know)
Lightning Strike
Whirlwind
Blind
Gale
Air Attunement
Aura of Restoration
When the fight started, i got up Aura/Attunement then just used Gale, Lightning Orb, Gale, Lightning Strike, repeat. It died in about 10 seconds or so, it was sad =P.