ITT: How to break players out of the player mentality of SHOOT, RELOAD, SHOOT, XP + LOOT, REPEAT and inject some genuine OH SHIT into the game.

Playing DH today, had my characters breaking up the corrupt cabinet of a planetary government. They had masqueraded into the Governor's spire, and had broken cover to kill most of the corrupt elements while they were in a meeting (explosion tore out one of the spire walls). PCs decided to head back to the lifts, planning to eliminate the few guards along the way and escape into the hives below.

They reach the elevator foyer, and are on the third story balcony of the room when guards open fire from next the lifts with their bolters. PCs return fire, so I have an elevator come up with four more guards.

PCs shrug. Then the next elevator arrives. Five more, all carrying Bolters the same.

PCs - "Huh, this'll be a tough fight."

Behind them, eight reinforcements arrive from the direction of the ruined meeting room and open fire with Lasguns. PCs have to split up and take cover behind white marble planter boxes. Guard casualties aren't too high, the Feral World gunner takes a good hit to the shoulder but stops the bleeding.

PCs are worried.

Elevator returns again, with another seven guards, some with grenades.

"Dude!"

More reinforcements from the interior.

"DUDE!"

PCs are sweating for a moment before one voices his thoughts. "We'll have an extraction soon, just keep them busy."

Two more rounds, one of the PCs goes down from a grenade and is stunned for six rounds.

"Um... any minute now guys..."

Elevator arrives with six Arbites from street level, all wielding riot shields and bolters.