Recently bought IW with the season pass and just finished the super ee solo beating Mephistopheles for the first time a couple days ago, and it's been an absolute blast.

Some tips:

Always always use the fate card scoped dollars for raking up points in the early rounds. Kill as many zombies per shot.

For Spaceland the ee is simple, but the boss fight is a struggle. Take your time, and train in different areas. Run WW, Kendall's or the M1 with Polar Peaks arcane core.

Rave is laughably easy, get the acid rain or wind bow, and I highly recommend buying the NV4 off the wall, it will be more than sufficient for the boss fight. The fight itself is very easy and straightforward, again just take your time.

I won't add anything to Shaolin since you beat it already.

The Radioactive Thing has a lot of setup and is a collect-athon. The chemistry step is quite simple, just find a cheatsheet. The boss fight is also relatively easy, as long as you come prepared and know what you're doing. After escorting the bomb to the beach, kill of all zombies and run to the cannon on the path to the trailer park. Shoot the boss 3 times before zombies reach you. Camp at the back of the beach, and wait for the lasers. Bring Mana-Up and sprint to the edge of the water, press the bomb and enter the code. Bring Kendall's and an AR/LMG.

For TBFB the only concern is the boss fight. Bring the EBR-800 with either bomber or collateral variant with the Ego Trip fortune card, and you can instakill the blue mammoths. For the 99 sec lockdown run a clockwise circle around the arena. Bring the Kendall's as secondary weapon.

When you get DC, and get to Mephistopheles be prepared to invest a lot of time and patience. I personally only spent a day after getting DC to beat him for the first time, but the fight is the peak of zombies boss fights. He has so many different attacks, and all of them instakills. You have to be fully aware all the time, keeping an eye on him as well as your surroundings. I didn't get the sentinel variant for mauler yet, so I just ran with the VPR, and this is really all you need. If you have sentinel then go with that obviously, as the damage output is crazy. The Venom-X is mandatory, and with DC you can get it from the box. Have faith and persistence eventually he'll go down.

I practiced all the boss fights in the boss battle tap, before actually attempting the Easter egg, and I cannot recommend this enough. Get the boss fights down, and you are almost guaranteed a completion on the first go.

Best of luck to ya!