A Gift of Earth and Fire

If your attempting to do this achievement at 110 be forewarned that you have to dps Forgemaster Gog'duh slowly so he can summon both adds before killing him! If both adds are not up because you killed the Forgemaster too quickly when Magmolatus dies you wont get credit.

Easy for Battle for Azeroth 120 solo and beyond...

1. Attack Forgemaster Gog'duh but with your weakest attack.

2. He will spawn Ruination, do not attack or kill add.

3. Attack Forgemaster Gog'duh ever so slightly.

4. He will spawn Calamity, do not attack or kill adds.

5. Do not kill the adds.

6. Kill Forgemaster Gog'duh, do not attack or kill adds.

7. Kill Magmolatus while both Ruination and Calamity are alive.

8. ?

9. Achievement Complete.

For those struggling in dragonflight since their damage is too high even without gear:

1. Take off all your gear. (besides a "low" iLvL bow, wand, staff or w/e if you need it to cast)
2. Use the [item=97919] toy. (CAUTION! read the disclaimer below)
3. Deal damage to [npc=74366] until he reaches 40% - 60% health. (don't know exact amount)
4. By now he should have spawned 2 elementals, Ruination and Calamity, keep the elementals alive. Kill [npc=74366].
5. Phase two is about to start and [npc=74475] will come out, kill him whilst again keeping the 2 elementals alive.

Also if you plan to do the other achievement Come With Me If You Want to Live in the same run, then make sure to free Croman first, otherwise you won't be able to complete Come With Me If You Want to Live.

Disclaimer: The [item=97919] will shrink you, reducing your size, mobility and most importantly YOUR damage considerably. This being said, the damage of pets won't be reduced! So keep this in mind and dismiss them whenever you attempt this achievement. Don't forget not to use abilities if they have a summon macro built in.