It took me about 3 years to get this badge on one of my toons, granted I kept him at a low level the whole (Just wanted the mallet) but I finally got it a few months ago. The i16 changes makes it easier. Way I did it was I found lvl 50s that was just RPing and afk in Peregine Island and asked if they would mind teaming with me.
With the new SK system they were able to go about their business and not be near me while I could run around PI as lvl 49. I then found a Master Illusionists and kept hitting her and had her on follow. She would send out her pets and I would kill them and then just follow her around until she sent out more.
You don't have to kill the fake illusionists, you just have to hit them at least once. This made it easier since they disappear pretty fast. I believe I would get 6 pets per summons and you only need to defeat 100 now.
This did take an hour or two but the last twenty went faster since the guy I was SKed too was done RPing and went and found another Master Ill and was doing the same thing, so obviously the more people you have the better.
I believe the fastest way to get the badge is to do a hard Carnival mish with a few controllers and so they can lock down the multiple pets, but if you can't find a team and can't find someone who has a carny mish then the method I used does work, just takes long.
I wont be able to get on until maybe this weekend, but if you haven't gotten the badge by then then I'll help you out.
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@Animorph
@Animorph 2
Also known as;
Maverick, Living Phantom, Role-Player, Live-Wyre, Eagle Eye, Toy-Man, Cartoon, Jetfire, Reflex, Mer-Man, Spartacus, Step, Reaver...
there is a very easy way to do this now 
Course this depends on the toon but you can have others help you out
lmk if you want help with it
Issue 24 PPM Calculator // The Great Makeover: The Vindicators
The badge requirement was reduced from 500 Illusionists to 100 and all pets except Decoys count. I have four new heroes at fifty who ran their taskforces and arcs and were still sitting at numbers under a quarter or far less on the progress bar so I usually farm for the associated accolade.
One or two (depending on AT) can do this easily in under an hour. Adjust challenge settings to spawn Masters and take a Carnie mission. If the scanner/paper doesn't provide Carnies, just exit the zone and return to refresh. Enter the mission and clear a section of all but two or three Master Illusionists. If you get an office map, even better - drag them into a small room.
Depending on what your AT's are, just hold, brawl, soft pew, etc. them to spawn pets. Make do with what you have, you don't necessarily need someone with holds. My spines scrapper and ice tank hold their attention just fine. You don't need a tank or heavy damage dealer either, two defenders can work like Tsoo scorcerers to heal one another.
Once spawned, damage every pet (Decoys don't take damage). The pets don't have to die, just take damage before despawning for credit. If you have an AOE like spines quills or ROA or really anything, all the better. Just use something that won't kill the Masters. Rinse and repeat.
Thanks guys, I was hoping this one had changed but I could not find it -anywhere-. I knew they had changed to where the other pets counted too, but I didn't know they had lowered the amount.
So I'm off to PI! lol
'Eagles may soar but weasels don't get sucked into jet engines...'
No need to search all over PI to find a Master Illusionist only to have her run off into other spawns or be defeated by somebody else. Go to PI yes, then pick a Carnie radio mission. There is a good chance the final boss will be an MI.
Don't count your weasels before they pop dink!
Anyone interested in getting a group together to work on this badge?
Anyone who has done it before and knows the tricks would be very much appreciated...but I suppose we'll take anyone. lol
'Eagles may soar but weasels don't get sucked into jet engines...'