Serious Sam: Siberian Mayhem - How to Unlock the "Massacre Party" Achievement in Under Two Hours

If you’ve been hunting for one of the rarest achievements in Serious Sam: Siberian Mayhem, you’ve probably noticed that “Massacre Party” is a tough nut to crack. The description says you need to complete the entire game in co-op on “Badass” difficulty with an enemy multiplier of x2. Sounds like a long, stressful grind, right? Well, I managed to unlock it in just 1 hour and 36 minutes, solo, while listening to a podcast. Here’s exactly how you can do the same.

How to Unlock the “Massacre Party” Achievement in Under Two Hours

Step 1: Set Up Your Game Session Correctly

Even if you’re playing solo, you must start a co-op session. Go to the multiplayer menu and create a private co-op game. This is non-negotiable—the achievement only tracks progress in co-op mode.

Step 2: Configure the Right Settings

Set your game options exactly as follows:

Difficulty: Badass
Enemy Multiplier: x2 or higher (x2 is the minimum)
Other modifiers: You can enable additional ones if you want, but they’re not required

Important: You must complete the entire game in one sitting—from the first level to the final boss—without returning to the main menu. No breaks, no restarts.

Step 3: Know Who Gets the Achievement

Only the host (the player who created the session) receives the achievement. If you’re playing with friends, the host is the only one who gets credit. If you want to try a workaround, have all players join first, then restart the first chapter with voting enabled. But I haven’t tested this myself—let me know in the comments if it works for you.

Step 4: Rush Through the Levels

Your goal is speed, not completion. Here’s how to move fast:

Ignore most enemies. Only fight when absolutely necessary.
Follow the yellow marker. Don’t explore for secrets or side objectives.
Use the 10-second invincibility after death. When you die, you get a brief window of immunity—use it to sprint past enemies and reach objectives faster.

Step 5: Grab Essential Weapons and Upgrades

You don’t need everything, but don’t skip these:

Pick up every new weapon you encounter. You’ll need something to fight with until you get the “Burner” (flamethrower).
Optional: Grab the rocket launcher from a secret and one skill point from a side quest in the first level. This takes only a minute and helps early on.
Collect skill points and weapon modules that are on your path. You don’t need to hunt for secrets—just grab what’s convenient.
Prioritize the dual-wielding skill for heavy weapons (left skill tree branch). This makes the Burner absolutely devastating.

Pro tip: Open upgrade crates only when you’re invincible (after respawning) or after clearing the area. The opening animation is slow, and enemies can interrupt it, causing you to lose the contents.

Step 6: Dominate with the Burner

Once you reach Chapter 4 and get the Burner (flamethrower), the game becomes trivial. Equip it in both hands and forget every other weapon exists. It melts everything, including most bosses. Save the cannonballs for the final boss—they’re the fastest way to finish him off.

Tips for a Smooth Run

Lower your graphics settings if your PC struggles. The x2 enemy multiplier increases the number of foes on screen, which can cause frame drops or crashes. Better safe than sorry.
Refresh your memory before attempting this run. Play through the game once on Badass difficulty, collecting all secrets and learning the layout. Knowing exactly where to go saves tons of time.
Check your peripherals. Right after I got the achievement, my mouse battery died. Don’t let something silly ruin your run—make sure everything is charged and working.

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Serious Sam: Siberian Mayhem – How to Unlock the “Massacre Party” Achievement in Under Two Hours