A quick FYI, the Road to Gears 5 Boost challenge (Complete Three Horde Games) is glitched so you can just load up Private Horde at wave 50, spam Sniper Strike it, and that counts.
Just to clarify (as if I haven’t clarified enough) - in terms of specifics it is the FINAL WAVE of a Horde match that needs to be completed so it’s not exclusively wave 50. If you set the Horde match to a 25 wave match like Juvie Madness or Boss Rush then starting and clearing wave 25 works too.
Sniper Strike spams are just a quick and easy way of doing it. Obviously it’s not essential if you don’t have Sniper Strike to a high enough level (or at all). Completing the final wave - 25 or 50 in ANY manner also works but may be harder and take more time.