I've heard he's really good but I have no idea how to use him effectively.
Some stages seem like they're designed to be impossible without her. Just set her somewhere enemies will walk by a hole, and if her skill is at least level 4 she'll shove them right off. A tank can help keep enemies locked onto the tile in front of her, if enemy volume is high. She can also be placed in front of your base just to stall enemies from getting to it before the rest of your units can put them down.
She's not really useful all the time, but some stages she just totally trivializes. The other push/pull units, like Rope, are the same way.
