I don’t think it is possible to get some of the early dungeon levels to three stars namely 2 7 13 14 15 20 and 22 but prove me wrong maybe you could try to use a non explosive fireball to kick shield boys around and to place them optimal but otherwise idk a way to get the levels with the 12 times multiplier

You can hit enemies multiple times to rack up the multiplier. The two only ways to break a multiplier is 1) you run out of time and the multiplier is filled with orange, and 2) your shot despawns and does not hit any target. (This is my speculation, feel free to correct me if I'm wrong) Something like this is possible. (level 2 3 stars one single playthrough)

The "multi bounce" fireball is just OP. (seriously somebody should name all the fireballs)

some of the maps being they are so spread out. try moving them all to one side and grouping them. from there ya should get multiple hits per.

3 shots before they disappear. so minimum of 4 needed.