Place an env_sprite.
Name: spr_red_green
Flags:
Start On: checked
Place another env_sprite
Name: spr_red_green
Flags:
Start On: unchecked
Place a multimanager
Name: spr_red_green_toggle
Turn off SmartEdit. Click add.
Key: spr_red_green
Value: 0
Click Ok.
Click add again.
Key: spr_red_green_toggle
Value: 0.5 (this is the delay between the flashes, in seconds)
Flags:
Multithreaded: checked
To make an explosion, use an env_explosion (doh), or use a func_breakable with an explosion magnitude value.
To make aliens float around, use a trigger_push, and cover everything where you want to have zero gravity. Then just place monsters. This may not work very well, since monster are not able to move when in mid air.