Alastor cursed under his breathe as he ducked back into the storage room, letting the fire sweep through the doorway between he and Vaisey before putting a shield between them and it when it struck and ignited the back wall. "No getting back out that way," he muttered, knowing if his assumption was correct -- his assumption always being the worst case scenario -- the shield itself would only hold for so long against the magical fire as it was -- and so long wasn't very long at all.
Coming through the back door had been the most sound idea. He and Vaisey had been first responders and they couldn't wait until back-up arrived from local sources or they'd lose their quarry. The front door was too open and they'd have been seen a mile away through the store window. The back was slower, but they could make up some of the time using his eye to survey the storage space before they entered.
Unfortunately, even his eye couldn't see through the strange shield of darkness covering the centre of the main store and they'd had to risk visibility to look around the door jamb to take stock of the room. That had been when the shield dropped and Voldemort saw them.
Nodding toward the cover of the store counter, he told Vaisey over the roar of the flames, "I go first, you follow and get a second shield across the doorway between us and that fire. Don't waste your time conjuring water." Hopefully the shields would buy them enough time to get Voldemort and then they could deal with the flames.
Firing off a quick succession of hexes as cover, Alastor barely waited for Vaisey's affirmation of the directive before making for the counter.