Firstly, u need to kill 4 hidden frogs, den u'll get " Q U A S "

den u will fight wit 1st boss
[attachmentid=319148]

After tat, find another 3 hidden frogs, den u'll get " W E X"

den u will fight wit 2nd boss
[attachmentid=319149]

Lastly, find de last 6 hidden frogs, den u'll get " E X O R T T"

den u will fight wit 3rd boss
[attachmentid=319150]

when 3rd boss is dead, Kael will be spawned

Just type -apntnmnb (All Pick, No Top, No Mid, No Btm) so you won't get any creeps too.
I manage to kill invoker with a more budget style, which is Soul Keeper's Sunder and Necrolyte's Reaper's Sycther.
Try it out, its fun thou to find out different way of killing the ultimate Invoker.