Very intriguing concept. I was excited about the idea of finding combinations that are effective and sending them to battle. That said, I couldn't really figure out how to make them work. I tried several times to make the mega legged bot. :-/
Two suggestions of potentially lightweight changes that could improve the experience:
Make the enemies much slower to spawn. Maybe even spawn them only once the player has spawned something. That would give time for the player to think and not be overwhelmed.
Allow the player to double click on a brick to move it into/out of the recipe. Having to click and drag (particularly with the possibility of missing between the slots, and thus having wasted the click) compounds with the stress from the fast moving enemies.
You've got a neat idea here, and a lot of the parts are almost ready. Well done setting up an exciting foundation. :-)