Well, the title really says it all.
Make it an ability spellcasters get at between 1-10 and make it masterable.
Secondly give classes an improved dodge/parry/block etc. that isnt masterable. The way I see it. The more proficient you get (lvl increase)the more opponents you should be able to handle at a time.
Itgoes something like this:
Tier I (masterable) lvl 10ish: Works against one opponent.
Tier II (not Masterable) lvl 40ish: Works 100% against the opponent you're currently engaged. You incur a 20% penalty against one other mob.
Tier III (Not Masterable) lvl 80ish: Works 100% against the opponent you're currently engaged. You incur a 20% penalty againstanother mob,and incur a 40% decrease in your ability to parry/dodge etc. against a third mob.