You should hunt down the technique of reusing raid lockouts week to week so that you only have to kill the LK.
I have a spare one in my inventory I’d give you if I could. I’ve had it drop three times between farming for the mount and grinding out my Shadowmournes.
It took me every week for eight years on a growing number of characters.
Ran ICC on multiple characters for years. I finally got Invincible on my nelf druid during last year’s timewalking raid event for ICC.
I ran ICC for many years on many toons, and this year it dropped for my DK with her having just under 100 kills.
Good luck. 
My guild leader just got it first kill right now…like wtf
Got it after 300 attempts over 12 characters per week
I got it on my 88th kill…
A pain, but less then the Ashes of Al’ar for me.
Kick this guy out of the guild!
I’m real salty worst of all he’s now circling me non stop on it…does that dismount thing work on allies 
I got it first try soloing as a noob. Not joking.
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I wish blizzard would do like 1 week a year event that increased all low rate raid mount drops to like 5% chance haha or am I just being a child.
I feel I’m addicted to farming mounts now , off to ulduar on my 5 toons now
The day we got our server first…
I remember telling my friend who was also running ICC for invincible “Watch the both of us get it right now.” And let me tell you, I nearly jumped out of my chair when I saw it drop for me. I was never expecting it to actually drop. What a glorious day that was. Too bad for my friend though, he’s still trying to get it! 
Same here. It took me many years of running it on my main and alts before it finally dropped for me.
Just started the weekly farm for this a couple months ago on 3 toons, after going for it casually over the course of 10 years on my two mains. I will have Invincible sooner rather than later, I can feel it.
I just do it casually now. Done the crazy runs of many alts, many times over the years, burnt me out.
Grats on getting it 
With the lockout change…
Selling precleared ICC - 20 available - Gold only - PST for details
lol
used to farm it like mad a few years ago but got board and started farming other mounts/transmogs