I've thought long and hard about this, and I think I've figured out the optimal PG placement to kill the onos.
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|O O
| O
|[][][]
That's one way, with the lines being walls and the circles being the phase gates. The bracket squares can be anything, be it a structure, such as a TF or arms lab, or another wall.
O O
O [] O
O O
That way, with the square brackets in the center representing a turret factory, the onos that attempts to kill the turret factory in an attempt to stop the numerous turrets sitting in base will have to set the rest of his body on top of a phase gate. That's almost a guaranteed kill.