Extra tips on Make 'Em Scream and Leveling Up to 50
I've gotten nearly all my scream kills from hitting enemies with the foam gun and sticking a grenade to them as low as possible on the foam so they go flying up. Regardless how they land, they normally got enough air time to scream.
The multipliers you see above the XP bar come up for 'Skill Kills'. Headshots and melee kills are the most common ways I see. Just string together some headshots and melees and you can get the 'max' multiplier easily.