You need to use all 6 types of equipment inside of the story mode.
- Breaching Charge
- Frag Grenade
- Incendiary Grenade
- Smoke Grenade
These are used as the last 2 slots in your equipment screen. You can switch them by holding and then using the left analog to select them. The fastest way to do this is to load up the Trainyard Scene of the level Mexican Border. Right on your left there will be a green container. This will allow you to switch your weapons freely. You need to use the 4 types of grenades, C4 and breaching charge. This short video shows the simple way mentioned above being put into action.