What you need to do to get the "missable" pyro's.
1. Free Laurentius. Buy his spells and ascend your flame to +10. If you acquire a number of pyromancies later and tell him where you found those pyromancies, he will search for them and go hollow. If he asks, simply respond "No" and he will remain a merchant permanently.
2. You must speak to Quelana just outside of the Quelag boss area BEFORE killing the Bed of Chaos and buy her spells. She will only appear if you have a +10 Pyromancy Flame. You can simply kill her when you have them for Great Tempest, or defeat Bed of Chaos and return for her to gift it to you.
3. Become infected with eggs to buy Eingyi's spells. Finally, offer 30 humanity to the Fair Lady to get the last pyromancy.
The remaining couple can be found in the world anytime before completing the new game cycle. The Black Flame obtained in the DLC does not count for the achievement.