Achak isn't the high-explosive type. He's the sneaky, traceless walk type.
He wants what's in the case. Armored or not, he doesn't want to blow it up with a high-explosive grenade.
He acknowledges that they are at a numerical disadvantage and that a surprise grenade attack could go a long way to even the odds.
If they go with a grenade attack, there are two approaches:
1) do it now before ponytail can see what's coming
2) do it after ponytail steps into the elevator, which could produce a chunky-salsa effect (although that puts the case at greater risk too)
That said, Achak's instinct is to let the man step in the hallway so that Achak can try to snipe him. If ponytail drops, great, maybe they'll stay undetected (physically, if not in the Matrix) for a few moments longer. If Achak misses or doesn't finish the job, Sister Rebecca can drop the hammer.
Someone remind me: did Sister Rebecca turn wireless off, or did she reboot? Is she back online? If she's wireless off, she might have to fire a preemptive grenade, as she won't be able to control the detonation as effectively without wireless.
Assuming she has some wireless control, Achak's sequence of events is:
IP1: exit room closet and move toward break room
IP2: recall Fly-Spy (simple? complex?), comm directions to Sister Rebecca
IP3: comm more directions if he didn't manage to say everything he intended to, another Observe in Detail if nothing else has developed
I realize that this leaves the decker a couple turns to get frisky and/or raise a general alarm. If that happens, we adapt and adjust accordingly.