Teleporting in and taking out key defenses is definitely a great idea given what the baddies are working with. Some spells that would be particularly helpful for this type of fighting are Earthquake, Freezing Sphere, Tsunami, and Transmute Rock. Tsunami is a particularly interesting one since it has effectively infinite range. If your Shaman is a high level swamp druid (very fitting for a lizardfolk shaman) than he might be standing way back at a vantage point with the main army and then making a tsunami in order to push away and corral any defenders who are grouped up enough to be a threat after the first wave of bad guys goes in. Anyone with Earthquake or Transmute Rock can use those spells to destroy the fort, walls and foundations of the towers. Freezing Sphere isn't a ton of damage but it's more than enough to kill commoners, guards, or low level adventurers, it's a very big AoE and moreover it can be regularly cast from 300 ft or cast into a marble and then dropped from a great height (way out of bow range). So tactical use of these spells, especially in conjunction with fiends and dragons and such can make pretty quick work of the defenses as laid out.