So I unlocked the 4th ER and didn't realize that my left flank was exposed when the play area increased. My next opponent showed my Achille's Heel as he used 4 Heavy Choppas, 4 Rocket Choppas to slice and dice up my defenses with ease. I was LOL watching the replay when I noticed that his HCs have the bonus of paratroopers as Heavies parachuted to the ground.
Thank you for the harsh lesson as I redesigned my base to put up more resistance to the OP HC-RC combo. I still lost a few times but at least one ER was left standing. Luckily for me, I needed to kill 3 more ERs to unlock the HC which I did in less than 10 minutes.
Since switching to the HC-RC combo, I won 8 of my last 10 battles. My opponents also used the same combo but they won by beating me by an average of 5 seconds.