EDIT 3: - Thanks AGAIN to Piyi Piyi for an amazing video showing how to deal with Guardians and how do the boost considering new updates! Check it at the bottom of this solution...EDIT 2: - Thanks to Piyi Piyi for some tips considering new classes skills!EDIT 1: After 2017, february 7th update, headshots made by Sniper Strike hability don't count for Headshot I bounty card anymore. You still can earn a good amount of points without the bounty card (about 8,500 PXP) by playing on the new Inconceivable difficulty.WHAT IT IS: A boost method using Sniper Strike Level 4 skill. This is the fastest way to boost your Player XP and at least Sniper class at the same time. For the others classes, the best way to level up fast is to play during
Double XP Events on
Developers Playlist. This playlist becomes avaliable for
Season Pass owners and their friends in the lobby everytime a new map is lauched (once a month).
HOW LONG IT TAKES: About 3 minutes, including loadings to start the match and to see the scoreboard.
WHAT YOU RECEIVE: It may have changed after so many updates!!!
> About 8,500 PXP = 170,000 Player XP/hour
(Sniper player)> 720 PXP = 14,400 Player XP/hour
(Other player) > 496 CXP = 9,920 Class XP/hour
(Both players) > 74 Cr = 1,480 Cr/hour
(Both players)> Plus 500 Cr after leveling up 5 Player levels
(Both players)WHAT YOU NEED:> Two controllers OR one friend
> Sniper Strike Level 4 skill card
HOW TO SET:> Join a second controller or invite a friend. Don't use guests, make sure it's a silver or gold account, so the progress will be saved, even if it's a dummy account. It will make the process easier and faster.
> Go to HORDE > HOST PRIVATE MATCH, select
Map: FOUNDATION
Difficult: INCONCEIVABLE
Wave: 49
Class: SNIPER for you; anyone except Sniper for your partner.
HOW TO DO:Go to the Fabricator, pick it up and drop it to make the countdown start. Now take it again and move it to the covered area near you.
If you have a second controller, use it to take the other player to the Fabricator and make a deposit. You have at least 54,500 Energy Points now.
As soon the wave starts, use the Sniper Strike skill. Stay near the Fabricator to receive Energy for your kills. It will grant you enough energy to one more strike at the end of the wave.
Wait 4 seconds (usually it is when The Quint ribbon appears) and use it again and again until the wave is completed. Always wait 4 seconds between strikes, to receive the maximum amount of XP. If you don't, you may waste 5,000 PXP due ribbons not being awarded, because you're keeping the same killchain. When you kill the last enemy, pause the game and hit return to lobby option. Wait at the confirmation screen (YES / NO) until the score board (WAVE 49 COMPLETED) appears. Now you can hit YES and leave. Even if sometimes the game shows 0XP and 0cr, you have received. You can check it at Warzornal and Store.
EXTRA TIPS AND UPDATES: I use Magic Bullet (lvl 4) for Sniper, and I have less problems when Guardians appears in the wave. And for the 2nd account, use a Scout with bonus for deposit, cloak and every HP boost card. When I'm DBNO, I play with the 2nd account (Scout) and I can revive me 99% of the time. And I play wave 50 until I spend all the energy (a little extra EXP)
Piyi Piyi made a video showing this method on Inconceivable with Magic Bullet (bottom of this solution).
If Sentinel spawns, you have these scenarios, from better to worse:
1- You can kill the Sentinel with 1 Sniper Strike (You can kill it only if the shields are down after the spawn)
2- You can kill it using multiple Sniper Strike (AKA playing as always)
3- You can kill it using several Sniper Strike (you spend more energy that a normal wave without Sentinels)
4- You can't kill the Sentinel and you receive less EXP because you fail the wave
- by
Piyi PiyiMAKE TURNS: When you reach level 10 with Sniper, ask a friend to do the boost with you in the same lobby. No friends? Use the dummy silver account that you have used so far and play with it, changing your main account class.
RESULTS: I guess it's the fastest way to work on the most grindy achievements at same time:

The Seriously 4.0 achievement in Gears of War 4 worth 1892 pointsComplete Insane Campaign, ribbons, rank placements, Horde maps, re-ups, classes and five skills FEEDBACK: If you noticed any mistake, want to give a negative or have any idea how I can improve this solution, please, leave a comment.
VIDEOS:Updated method (by
Piyi Piyi)
Original method (by me;
rafa1000house)