The limitation on big creatures is that if the PCs manage to get some stun and/or injury penalties in there, that may put a big foe off balance long enough that it never recovers (especially with stun since it also gives a bonus to hit the creature). So if you want it to have a chance, have a way to mitigate that. Could be high skill in Fortitude as Hurin notes, could be healing, stun relief abilities/item/herbs, could be immunity to stun, etc. Multiple attacks are also a good idea so they all need to think about defense instead of letting one person take the brunt (because if that person is doing a full parry or other extreme defensive maneuver, they could be ok).
Alternatively, have a few lesser minions to keep them from focusing completely on that single foe.