As you kill enemies, you progress towards earning the next multiplier level. This is displayed by the circle around a number at the lower part of the display. Missing one shot or taking damage will reset it to a 1x multiplier. Get the musket to at least level 3 and shotgun to level 1. Use the Musket almost exclusively. It should be able to one hit kill un-armored Dodgers and the jet pack enemies. When targets get too close, use the shotgun to clear them out. It can spray anything directly forward, and a lane to either side, and up to three rows in front of you. Accurately clear out enemies as quickly as possible. Focus on where they spawn and line up your shots with your peripheral vision. On occasion Dodgers will spawn alongside Grunts and/or Behemoths. Because of the Dodgers' ability to change lanes it is usually better to let them get within your shotgun range before taking them out. If they spawn alone, you may still find it easier to wait for them to get close. Use caution shooting tanks. When it explodes, it will spawn a Grunt on either side. With patience you should be able to reach the 10x multiplier at the end of wave 2 of level 7.