Make sure you can put multiple opponents on your PCS.. Then who ever the PC is attacking you put on Full parry while the other opponent Attacks at Full OB. If the PC switches targets cause the one guy is hitting him to much, Then have both opponents go to Full parry Until your sure the PC is concentrating on one then have the other go FULL ob again LOL...
There is a good skill to evade that, 'reverse striking', as you don't need to rotate to hit anyone, they can't know who is the target, so you bind all foes to parry or be in danger of a full attack with no parry. This, that makes foes to use half parry usually, plus an heavy armor (AT19-20, min. AT16 or AT18), makes you a very more efficient multi-foe combatant.
I think combat in RM is balanced, we play much time and we have no problems like that, I can say you about advantages of parry:
1) You can't now who attack first, so if you use full OB, but you lose initiative, you can die easily. Usually at first time you must parry, and if you make a critical that gives you initiative win next round/s, or foe must parry/stun, etc. Then you can use full OB to attack, but this is called 'take advantage in combat', and you don't have it from beginning.
2) You can fumble, maybe you can recover the fumble with swashbuckling skill, but in any case you lost your attack, so now you need to defend your foe's attack, but if you are not parrying...
This is the same if you do a very light attack, not fumble but rolls like 10-15.
So, to be really safe, you should only use full OB in cases like foe can't attack (stuned, must parry, etc.), and use less parry (maybe 20% OB) if you have sure initiative (foe earlier critical result), because you can fail your attack, and you have some defense.
For multiple-foes combat, use 'reverse strike' and always FULL OB, unless the foe in front of you is stronger than the others, in that case you can parry with some OB against that foe. And don't forget crossing your fingers.