It doesn't have to be the same target. Combos occur when multiple hits are made by your team without being interrupted by something else. So if the opponent attacks, dodges, gets off a mystery even it it doesn't hit you, the combo counter resets. Combo chases don't always hit but most of the time they do if you get the combo needed as long as you haven't hit the combo chase already. Unless it lets you chase more than once for some reason. (I don't think anyone does yet, but maybe in the future.) Knowing initiative and action order is important when going for the higher combos like Roshi. Unless you can for sure get off 30 with just automatic chases.