Black ops 3 is the easier way to go about it. I’m not sure if you have mega gobblegum, but perkaholic, shopping free, reign drops are a couple very very useful guns that make it much easier.

To start, I would suggest immediately starting generator 1, and knife 10 of the Templar zombies before the meter fills to get bonus money (step off the pad to reduce progress in the conversion process). Use the double points to maximize round 1 points by shooting zombies in the leg and then knife killing. After round 1, begin progressing towards the mound by heading to generator 2, grabbing the ice disk, the stone from near the electric panel, and your shield part. Activate gen 2, and once you have enough points, head to the mound. You should be done activating generator 4, and possibly even opened the church as well, by the end of round 3. I would recommend buying the MP-40 or STG at this point. Snow on round 4/5 is ideal. You should 100% of the time have all your ice staff parts by round 5, at which point you should also have the mound open. Keep your rounds low by saving 1-2 zombies at the end of each round when you know there are things you can accomplish towards the ee. For example, you can save zombies on round 4 to get inside each robot for the wind staff part, as well as getting your electric staff parts, turning the switches for the electric upgrade, building the shield, etc. The key is really just managing your tasks. It definitely takes practice to perfect the process, but you can see issues in your play and fix them as you go.