You can use a single piece of Avenger Gear and Power Activator to play in basic multiplayer mode.
Use the following steps to set up your Power Activator for basic multiplayer mode:
When you first visit a location and haven't yet selected a mission, you will land in Exploration Mode, where you can use the Power Activator as a firing unit. While in Exploration Mode, you will be able to hear background noise from the location you're in.
Press the gold button on your Power Activator to spawn an enemy.
Press the red button to fire.