The red zombie should walk left and be shot dead by the grenadier. You do not need to worry about him.
Collect the Miasma power-up and use it to block the vision of the grenadier on the right, above the mallrat in the blue suit.
Assimilate the mallrat in the blue suit.
Before the Miasma wears off, send one zombie back to the small room where you started, and the other one right, then upward, to collect the Screech power-up.
Use the Screech power-up on the zombie in the small room. Make sure its effect reaches the Man in Black.
Use your other zombie to assimilate the grenadier southeast of the Man in Black, then go into the small room and assimilate the Man in Black.
Collect the three disguise power-ups and the Miasma power-up.
Disguise one of the zombies in the upper room, and send him to collect the Phantom and Disguise power-ups, and if possible the Miasma power-up, in the room with the Detective. Do not assimilate anyone here, and if you do not think you can get the Miasma power-up, just get the Phantom and the Disguise, then leave.
Send the same zombie in upwards, in front of the closed doors.
Alternatively, after collecting the Phantom and Disguise power-ups, also go and collect one of the five Tough power-ups and use it on your zombie.
Stand as close as possible to these doors, then use the Phantom power-up on the zombie. This should get rid of the disguise, and the grenadier will shoot the zombie with a grenade, opening the doors.
If you had chosen to collect the Tough power-up, your zombie would have survived being shot, and possibly the grenadier may have been knocked down by his own grenade. If so, assimilate him. If not, after the doors have been opened, move closer to the grenadier so that when he shoots you the blast of his grenade knocks him down. If this happens, you save one Disguise power-up, even though it is not needed.
Make sure that both doors have been blasted open. Your horde cannot fit through if only one door was destroyed, and if this was the case, you will have to restart.
Disguise another zombie from the room that had the Man in Black, and send him to assimilate the grenadier who opened the doors. If you had used the Tough power-up earlier on, just proceed to the next step.
Send the same zombie upwards into the small room above. Assimilate all 8 humans here and collect the Disguise power-up.
Disguise any two zombies and send them to assimilate the remaining two grenadiers in the main hallway. You will lose one of these two zombies.
Use your final Disguise power-up on any zombie, then send him into the room with the Detective through the top entrance.
Use Miasma to block off the bottom entrance to the room, then assimilate the Detective with the disguised zombie.
Assimilate as many humans here as you can. Send the rest of your horde if necessary.
Once you think you have enough zombies to swarm and assimilate the two policemen on the right, gather your entire horde near the red carpet below the room with the five Tough power-ups, and collect the five Tough power-ups if you think they will be needed.
Quickly use your Miasma power-up to block the green door, covering the lone suit person in purple.
Swarm the two policemen.
Swarm and assimilate any humans who escaped the room with the Detective, as well as the three suit people in purple.