Oh man, I am so sorry to hear coryza (aka BIG EVIL!) is visiting you. I had to put down several hundred birds last summer....very sad.
Tylan is definitely the way to go. The safest way to do it is injecting into the skin at the back of the neck. The bummer is that sometimes the feathers are too dense to really get to the skin easily OR the needle goes all the way through and out the other side of the neck (not good....) OR the fluid pockets up and then oozes back out. When I get frustrated I lay the bird on it's back in my lap and run the injection next to the keel.
I had it last summer and put down nearly everyone. However, I kept a handful of survivors so I vaccinate new birds. It's a total drag but it seems to be working.
Good luck.