No level 25 testing. Your Inferno Lamp is only consumed when you win. After that, you have to pick up the quest again from Louisoix and get a new Inferno Lamp made. Currently there's also a display error where the chat log says you hand over an entire stack of Inferno Tapers, but really it's just 5 more.
Also, tapers shouldn't ever be an issue with the occasional spiritbonding/coffer-farming at Zanar'ak.
We count phases like this:
Phase 1: Ifrit only performs auto attacks
Phase 2: @80%, Ifrit begins using abilities
Phase 3: @50%, Ifrit glows, begins using abilities in slightly shorter intervals, activates Sear (AoE pulse for ~250-350 damage). The behavior of many abilities changes.
Phase 4: @30%, Ifrit summons four Infernal Nails.
Phase 5: @60 seconds after Phase 4 begins, Ifrit uses Hellfire, then resumes using his abilities in even shorter intervals than in Phase 3. The behavior of many abilities changes again.