Bloody Mess

Easy Solo for Level 70 Dragonflight, Level 80 Worldsoul Saga & beyond...

1. Normal Difficulty.

2. Begin fight.

3. Nuke boss down to phase 2.

4. Hang out and wait until you have 12 bloods around you, you'll see the organs casting from the sides of the room and 3 will spawn each cast.

5. Once you have the bloods all on you, massive aoe and nuke them down within 3 seconds.

6. Take out the organs and nuke the boss.

7. ?

8. Profit. Achievement Completed.

For the avoidance of doubt, and to help prevent solo achievement hunters from checking out YT videos; if you are wondering why the [spell=311160] are not spawning, despite quality advice in these comments, then read on.

The 'phases' being referred to require [npc=158328] be 'killed' fully four times before the fight is complete and the boss is 'downed'.
So, if you are tip-toeing around, trying not kill him (as a ridiculously OP, War Within or higher player) by accident; fear not!

Kill him once, slay an organ at the edge of the room. Do it twice more (3 [npc=158328] kills) and the required [spell=311160] will start spawning rapidly.
Make sure you have a dozen or more (to be sure you definitely get the required 10 in the 3 seconds.) Let loose with all your majestic power, kill 'em all, then down Il'gynoth for the fourth and final time. Open carton of Ben & Jerry's, feel smug.

I recommend doing this in the first transition phase since you won't have the boss or beams to worry about, and if you end up needing to wipe, you won't have wasted as much time. Let the organs cast and then kite them around the edge of the room in a circle until you have a few over ten bloods (to be safe). When you have enough bloods, begin to spiral into the central pool. During intermissions, you will not take damage here. Wait until [spell=311160] expires so you do not take excess damage while you burst, then pop hero and your defensive CDs. Don't forget to kill an organ to continue the fight as usual.

Note: The bloods will gain [spell=317672] if they are not killed in a certain amount of time, making them unable to be CC'd. I'm unsure of the exact time, but by the time the third wave was about to spawn, the first wave of bloods had all gained them, so it will be impossible to use Gorefiends' Grasp or Ursol's Vortex to catch all of them when you are going to kill them.