EDIT - THE AUTHOR HAS UPDATED THE TRAINER - DO NOT USE THIS METHOD (some functions weren't working anyway).
So, I figured I'd put this together for folks who can't find the deprecated files to make trainer work on current systems (with a Windows PC). I've personally tested it this week to ensure that it works. I have listed a few steps as optional, here's what this means. When you cafiine without a server option, it will AWHILE to eventually timeout, before continuing on. This means you'll be spending a good 5-10 minutes literally watching the WiiU and/or game title boot screen until it times out. So, if you don't feel comfortable running the exe, then you simply skip these optional steps - just be prepared to wait - it WILL eventually boot.
1. Extract the geckiine.zip to your WiiU sdcard.
2. (Optional, but recommended) On a Windows PC, open a command prompt window and type: ipconfig. Take the IP address (likely a 192.168.x.x or 10.x.x.x address) and write it down. You'll also want your WiiU's IP address, but you should know how to get that if you're here using trainers.
3. (Optional, but recommended) Extract the CafiineServerUI on your Windows PC and run the executable inside (feel free to virus scan on virustotal, it's clean). It should run and say it's listening.
4. On you WiiU, run the Homebrew Channel and run geckiine.
5. When geckiine runs, you'll be asked to point to an IP address. If you're using the optional steps, point to the IP address of your Windows PC that you got earlier. If you are skipping these steps, just hit the button to continue with the default address (and wait, wait, wait).
6. You'll be kicked back to the WiiU menu. Boot the game (Note: I have NOT tested this with a disc based version of the game. YMMV). Once again, if you skipped the optional steps, be prepared to wait, wait, wait. It WILL eventually boot.
7. Run the trainer on your Windows PC, and point to the IP of your WiiU and connect. It should connect successfully.
8. Additional Note: Keep the CafiineServerUI running until AFTER you quit the game and are back in the main menu. To completely unload it (and skip the timeouts) just re-run the Homebrew channel and/or reboot the WiiU.
9. Enjoy the trainer.
Let me know if you run into any issues (or I missed a step, I'm writing this from memory). The steps are similar to others listed in this thread - I just consolidated it and added the files directly since some of the links were dead. Mods, let me know if I fucked up attaching them here and I'll host them elsewhere.