I had an ugly battle go down in that same area. Before that battle, I did one terror mission on that map with Sebillians and Reapers. All of the lizards hid behind the wall the entire time. Was a bit boring and I remember thinking it'd be nice if they got aggressive. Wish granted.
So on the last mission, I'm expanding outward towards the picnic area, bus and gas station expecting the same thing. One Sebillian and Reaper down. Coast is clear so I hit end turn. 5 Reapers, about 10ish Sebillians, 2 light drones and one of the bigger ones show up. I couldn't retreat back to the shuttle fast enough. 2 soldiers go down immediately and a few others should have too without some dumb luck and a little smoke. Reapers run all the way up to me but don't actually kill the soldiers. I get the other 6 on the shuttle with everyone hurting.
At that point, a few Sebillians are feeling pretty good and strut towards the middle and I am able to pick them off. The turning point finally was destroying the larger drone. The explosion took out a light drone and a 2 Sebillians. It was ugly, fun, but also made me regret slightly wishing they'd be a lot more aggressive.