One tip: Against characters that have multi-hit attacks, you can press and hold Shield for the first hit, then tap Shield (to get the EX Shield) for the second hit. For example, Satsuki's crouching C sweep is two hits, so if you sit down and hold D to Shield, you automatically Shield the first hit (but it won't trigger Last Arc). Then right after, you TAP D to EX Shield the second hit (which DOES trigger Last Arc).
Note that some characters have Last Arcs that can only be done in the air. Arcueid, Kouma and Ciel are good examples.