He briefly mentioned about creating artificial magnetic poles to shield the Mars city from radiation. No details though. Maybe we could have learned about it the Q&A session was a little better.
Musk tried to steer his presentation back to the mission of SpaceX which is focused on transportation and space. The scope of travel is already too big so terraforming and the questions regarding what/how life will develop is begging for another organization to tackle.
I was thinking about this the other day, and the best solution that I came up with was to hit Mars with Ceres in hopes that the collision will be sufficient to restart geological activity on Mars. Sure we have to wait a while for things to cool down again and coalesce, but it seems a much more feasible solution than living in bubbles or trying to create an artificial magnetosphere.
Wonder if a nuclear bomb "The Core"-style would work? (Yes, I realize this is a shittt sci-fi movie, but if you are taking about smashing a dwarf planet into mars to generate a huge explosion it doesn't sound too out of the realm of workable.)