Play Level: Decima on the Veteran difficulty with the Nemesis ship. Only focus on keeping the multiplier active and avoid clearing the area of enemies too fast as you need some enemies to shoot to keep your multiplier alive. So leave some enemies on the screen until the next wave appears. Conserve your bombs for Phase 2 and Phase 3, and save Overdrive to use against the third version of the Boss. Clear every minion cube until just one tiny grey box remains. When the Boss spawns a third copy of itself (it will start flashing red), use Overdrive. If you do not have a 15x multiplier by this point in the level, go to the next level to continue the multiplier and get it to 15x to get the "(10+1.5+3.5)x" trophy. Note: You can also use this method on Level 1: Phobos.