There is no order, it all happens at once. Because of this, the game uses LKI (last known info), meaning the game "looks back in time" to see what was on the board when the effect kicked in. Since there was a Hissing Iguanar, its ability triggers for each dead creature (including itself). Notice, however, that the ability will only be put onto the stack when SBEs are checked, so if you had a Hissing Iguanar and 3 2/2 elves, an Imperious Perfect and 4 3/3 Elves, when Pyroclasm resolves SBEs will be checked. It'll see Iguanar, 3 2/2 Elves and an Imperious Perfect, all with lethal damage on them. They'll be destroyed, and the Iguanar ability will trigger 5 times. Then, SBEs will be checked again, and the 4 3/3 Elves will now be 2/2 (Perfect died), and will thus have lethal damage on them, ALSO being destroyed (but now NOT triggering the Iguanar). THEN, SBEs will be checked one last time, nothing worth notice will be afoot, and THEN the Iguanar's ability will be put onto the stack.