Heroes Phantasia from Namco Bandai blends together the popular Sgt. Frog series with the girls from Read or Die and the "annihilating second bullets" from s-CRY-ed into one PSP game. Altogether, Heroes Phantasia has ten series and over a dozen familiar characters from Darker than Black, Blood+, My-Hime, Slayers Revolution, Sorcerous Stabber Orphen, and Rune Soldier.
Read about the game’s battle system here and you can see more screenshots of crossover bosses in Heroes Phantasia here.