Well, there is gear out there that adds exponentially to your discipline, where, even a char which only gets 3 to four points for skills per level can maintain their footing quite easily in cases of say, knock down, shaking mountains, and so forth. Having some pvp experience and knowing the limitations of a level 20 char, plus the gear which I have seen, I don't think that one would have any issues making a char, with or with out gear that can stay on their feet if that is the goal of said char. Balance is quite simple, everything has a weakness.
For most melee classes its usually will or reflex, for most ranged its usually discipline, fortitude. Etc and so on. If you are a range char and you fall on your rump and you don't want to then find the gear or spend the xp to get the gear that is enchanted and boost your discipline or spend the skill points. Will one thing possibly be sacrificed for that goal of staying on your feet, probably. As a melee char you really shouldn't have any issue putting points into disc, I would imagine such would rather be something most would do as they would want to use their greatest assets of knocking down their enemy and thrashing them silly while on the ground before they can get a spell off or any other thing which might impair ones life.
If you're talking about bolt attacks, discipline seems to have no bearing on them, its whether they hit or they don't, in which case you might want to be worried more about ac or using something as simple as a freedom spell or potion. Everyone can get/make potions, so its more about the planning when it comes to that.
I think asking for an immunity to discipline is rather like saying a vampire, a fleshy undead, cannot be turned into a statue with flesh to stone. It makes no real sense on a level 20 server.
Of course, the real trick might be trying to figure out what the weakness is of the char you are trying to pvp, one of my chars has quite a few despite looking very strong in most area's. From piercing to a low to hit, it makes the char quite vulnerable in combat with out the magic of her friends. Travel with people who balance you out, figure out what you want your strength's and weakness's to be and then find your own balance some how.