Well, first I tried to kill the blue early and this started to get out of hand, I kept most of my cells close to the core. It is almost mandatory since blue can spawn really close. As for upgrades, whenever it was possible, I always chose the heal and the ones which spawned more ranged cells. I didn’t have an other try with this after, so I don’t know how bulletproof this strategy is, I guess I had some luck too.