Let me see if I can stay coherent...
5C, 236C, and the like are all fighting game terminology for the various moves you can perform. In SWR, the buttons are as follows:
A - melee attacks - the Z key
B - weak danmaku - the X key
C - strong danmaku - the C key
D - dash/flight - the A key
A+B - switch your cards around - the S key
B+C - use a skillcard/spellcard - the D key
As for the numbers, they refer to directional inputs. Assuming you're Player 1/facing to the right:
5A - standing still, pressing Z to attack with melee.
6C - holding the right arrow key and pressing C to attack with strong shots.
2B - crouching, then pressing X to attack with weak shots.
236 is a directional input that requires you to move from the down arrow, to holding both down and right at the same time, then just holding right. ...I'm not sure if that came out clearly or not. :ohdear: Anyhow, in Yukari's case, doing 236C with her default skills would cause her to drop a gravestone from the sky.
Going back to Iku's card, using Yukari's stationary strong shots should allow you to bring down Iku's shield early. ...On Easy, anyway. Does that help?