The new RMU rules are good for this:
--Shield Bash has its own attack table, with primarily unbalancing and Krush crits. I think RM2 might have had a shield table too in one of the companions?
--Shield is its own skill. I think the developers want players to have to develop skill in bash separately from skill in defensive use, though our group just allows one skill to serve for both
--Shield Bash follows the normal rules for actions: a full attack with a shield bash would be 4 AP, a -25 attack is 3 AP, and a -50 attack is 2 AP. The Multiple Attack skill can be used to reduce penalties if you want to both shield bash and attack with a main-hand weapon at the same time.