When a commander designates you and your squad to be OPFOR (Opposing Force), what they're doing is giving you an opportunity at the most fun you can have in training — playing bad guy.
This is a way for you to use all the knowledge and dirty tricks you've ever learned to put other troops in your unit through the ringer.
The purpose of this is to give realistic training to test the unit's knowledge and metal so your commanders can figure out where the faults are and how to fix them. While being OPFOR is still training to a degree, it's a great way to skate in the field and get the hell away from your platoon for a couple hours.
1. Be aggressive
Your goal as OPFOR is to ultimately "die." The unit you're fighting against will have a mission and a plan, which typically end in their victory. Don't let that get you down — you still need to put up a good fight. Don't just hand them an easy victory. The point is to give them some good training; so put them through hell so they can learn something.
Match their aggression. (U.S. Air Force photo by Tech. Sgt. Sam Weaver)
2. Be deceptive
Deception is key in any form of a defense. Your goal is to fake out the enemy to make it easier for you to wipe them out. If you're unpredictable, the enemy's life will be much harder when they come after you. In the case of OPFOR, you'll already know what you're defending so make sure to lead your "enemy" through a big maze.
Use cardboard cut-outs and robots! (U.S. Marine Corps photo by Lance Cpl. Ashlee Conover)
3. Use their tactics against them
They're your unit, so you understand their tactics and standard operating procedures, which gives you an edge that a real enemy won't have. You know what they're going to do in any given situation so you can provide a perfect countermeasure. When evaluating your unit's SOPs, be sadistic in your planning to give the ultimate defense.
If you know they're going to climb over walls, booby trap the walls. (U.S. Marine Corps photo by Lance Cpl. Eric Tso)
4. Use your environment
Urban areas are filled with tons and tons of props. Training sites will likely imitate this and place old furniture all over the place, and if you're training in an abandoned housing area, the chances of this will be much higher. If there are doors around, set up barriers or obstacles. Make your enemy work for their victory.
See that car? There's a lot for you to do with that. (U.S. Marine Corps photo by Lance Cpl. Sarah N. Petrock, 2d MARDIV Combat Camera)
5. Use every weapon or tool you have
If you've got para-cord/550 cord with you, use it. Set-up as many booby traps and trip-wires as you possibly can to increase the level of difficulty for the guys trying to get to their objectives and accomplish their mission. If you have smoke grenades, oil, and/or trip flares, use those to the most frustrating extent possible.
Don't be afraid to use one of these bad boys if you got one. (United States Marine Corps photo by Lance Cpl Samuel Brusseau.)
6. Employ unconventional tactics
The use of unconventional tactics dominates on the modern battlefield; when you're OPFOR, it's a great opportunity to toss out the rule book and mix your conventional knowledge with unconventional tactics to kick some serious ass.
Fight aggressive, fight dirty, and be deceptive. Fight to win and give the guys in your unit a real challenge to test their steel. If you manage to beat the hell out of them, it only increases the amount of fun you're already bound to have playing bad guy.
A well-planned, well-executed ambush will inflict devastating casualties. (U.S. Army photo by Lt. Benjamin Haulenbeek)