I was trying to attack a portal today, but I couldn't destroy the last few resonators because on the map they exist in the middle of a city block. I tried going in the building, but the gps signal is gone at that point and it didn't get me any closer to those resonators.
How do I destroy resonators I can't get close to? Do I just need to launch a ton of xmps from a distance and do around 1% - 2% damage at a time?