Shildas look like upgraded versions of Shiidas, but they actually fight quite differently. They'll still waste turns by using Wall Guard from time to time, but what they really like to do is hit you with Toxin. Because they come in such huge groups, they can easily inflict your entire party with Toxin; and it will do HUGE damage each turn. Considering it'll probably take two blasts with Aquarus to drop just one of them, and their Toxin damage will be adding up, these are among the most annoying enemies in the game if you don't have the proper equipment to negate Toxin.