Shattered

In order to obtain this achievement, you will need 2 constructs alive which are spawned/activated during the Ignis encounter. Kite them through the fire pits left by Ignis, and once the Heat debuff reaches 10 stacks, they will gain a debuff called Molten which lasts for 20 seconds. Then kite them into the water, which will give them the Brittle debuff. Kill both of them within 5 seconds and then kill the boss.

As of Shadowlands this one is bugged. Can anyone confirm? Tried it 2 times.

Earned this today, never changed any of my gear.

Pulled boss and ran around until he had spawned 2 constructs, then stood still and waiting until both constructs had the heat buff, then pulled them and the boss into the water on one side until lost their buff and gained the brittle debuff (it happens instantly) and then I just used whirlwind to 1 shot them at the same time as the boss, achivement popped with no issues.