Q u o t e: I think some people missed the point of this post. Of course a single element sorc is superior to a tri-element sorc. I, myself, like to use a lightning sorc with an infinity equipped merc when I just want to outright obliterate monsters.
What I was suggesting was a way to make a lightning sorc and get two other moderately useful skills in the other elements for free (i.e. without investing in all the pre-requisites.)
And for the people who say firewall is for newbs, obviously you don't play with a merc. Firewall kicks ass, even at a moderately low level, when you have a merc to keep the monsters in one place. It's not hard to aim firewall at a monster that doesn't move much.
Yes, the topic has moved a bit far from your original post. I believe I hinted at the answer above, but to clarify, you just simply won't have enough damage to compete well enough in A5 Hell without investing some kind of points into each of the trees. I say this, because I have tried doing something similar to what you proposed - basically a fire/cold build, but with a lot of +lightning/+lightning damage/-lightning resist stats to try and build enough lightning damage to down fire/cold immunes.
I tested this on an editor, automatically given all the necessary gear, and I still struggled to try to get it to work. Given legit play, like on a ladder reset with no dupes, you are not going to have access to all the facets, griffons eye, and so on, so even if I did achieve decent results with it, it is not a pratical build unless fully twinked.
EDIT: I cannot remember if I tested with infinity or not. Since I am never going to get a legit infinity, I may have not included that. However, I doubt it would make too much of a difference. Negative resists on little damage to begin with, is still little damage.