Thank you both for your message response to my question about this earlier.
I wanted to give you a status update - I was able to obtain the achievement on both platforms (Windows phone 8 and Windows 10 PC) with the following method:
- I had previously completed 89/90 challenges, and for my final challenge I needed 11 more promethean kills with a scattershot on mission E-5.
- Log into Spartan Strike on both platforms while connected to the internet and Xbox Live to sync all progress.
- Leave my Laptop connected with the game running at the main menu, and synced to Xbox Live.
- Put my phone into airplane mode and restart the game.
- My phone gave me an indication upon starting the game that I could sign into my Profile, but not connect to Xbox Live, and the data was being saved locally.
- Finish the final challenge on my phone, while not connected to the internet or Xbox Live - Achievement unlocked.
- Finish the final challenge on my laptop while connected to Xbox Live - Achievement unlocked.
- Close the game on my phone, and turn off airplane mode. Wait for WIFI connectivity.
- Restart the game on my phone, data now syncs up to Xbox Live.
- The data showed up on Waypoint for my Windows 10 completion almost instantly.
- I played a single mission on my phone and completed it (A-1).
- The data showed up on Waypoint for Windows Phone in about 5 minutes.
I have both achievements, and the time stamp worked as well.
I do not know what would have happened if I tried to do them both while connected to Xbox Live - my impression is that the challenges would sync, but the achievement would not unlock on both platforms.
This is likely still an issue for other gamers that would be nice to address, seeing as 99% of them probably wouldn't look into this before trying to complete 90/90.
Thanks for your time, and I hope you have had a good holiday and a Happy New Year!
Please feel free to lock this thread off, I have another in the support forums that might be used for people that still experience this issue.