I usually played with friends....did the EE every game.
We milk A51 for max points by having 1 person loop the zombies after the 2nd horn. everyone max's their bullet points in the horde and we go to the station where we save 1 zombie. You need the points to get to the dome.
We go tunnel 6....open all the way to the dome, hitting the radios, power, computers and splitting the hacker.
After a few rounds of earning points, we get the bowie and open tunnel 11
After doing the EE hundreds of times....this is how I do it every time and I rarely have to wait past 12 for tunnel 6 to hit.
What really sucks is if some noob stops the excavator from decompressing 6....I usually restart.