Took him down just a couple days ago. It ended up being about a 20 second fight because I gathered up the higher level characters in my guild, so we had about 3 groups of mid-50s with me, a then 40 bard, thrown in the mix. Strangely enough, the doppleganger looks exactly like Maligar--a human with a beard in a red robe. He did not cast blind, but he did fear. With buffs and MR song running his two attempts were resisted, but it's just an FYI. A hint: take out Maligar's buddies before giving him the note if you're gonna take him on with only one group. Granted, they're not difficult mobs, but having another 3 or 4 baddies beating on you is irritating at worst.
Soloed this guy at lvl 51, played lvl 9 song elemental Rhythms and my 2 Chant songs. I took less than 1 bub of damage against the doppleganger. I cleared all the other mobs before giving Maligar the note. I tried this also with out the lvl 9 Elemental Rhythms and the doppleganger hit me several times with a 400 pt zap. I zoned and came back with a different plan then he was easy.
On the Vazz server, I took him down this morning, I also did page 24 upper and lower within about 6 hours time, also including killing Quag, and Blackwing, and giving Forpar the note, basically putting a nice start on page 25. I had a full group of lvl 50 plus people, as soon as I gave Maligar the note, a lvl 60 dru, lvl 55 dru, lvl 51 dru, lvl 56 mage, lvl 54 necro all blasted him at once, he didn't even get the chance to try and fear anyone. While me, the lvl 52 ranger kept everything else busy, we cleared the camp while Maligar was charmed, and then medded up to 80m around the board, then I handed in the note. The doppleganger died almost instantly. So I honestly couldn't tell you how hard he is, but if you have good guild/alliance support, he's cake.