So the cable won't hold tension because the screw hole in the lever is stripped, so the screw can't clamp down on the cable well enough to hold it in place?
If so, get a knarp. (This one would probably work well, but feel free to look at the rest. A smaller one would look better, but I think those can be hard to use sometimes).
Then run the bare end of the cable through the hole as you normally would, but instead of using the screw to clamp down on the cable, just use the knarp to clamp down on the cable after it goes through the lever. The two main problems with this plan are (a) that it will look funny, and (b) you probably stab yourself in the hand with the cable a lot, since if you trim the end of the cable to much, the knarp won't work properly. (You may also need to trim the cable housing to get enough slack in the cable to make it work with the knarp.)
Or you could just replace the lever (I have no idea how that works though), or do this or this.
(I love it when I find the better solutions after I've typed out two paragraphs explaining the crappier solution.)