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As of Operation 4 the honor required to reach level 5 has been drastically reduced. It is now less than 1/10th the amount previously required to unlock the achievement and a huge slap in the face to those of us who earned this pre-update.
The new levels are as follows
LVL 1 Needs 25 Ally XP Totalling 25 Awarding 2% XP Bonus
LVL 2 Needs 125 Ally XP Totalling 150 Awarding 4% XP Bonus
LVL 3 Needs 250 Ally XP Totalling 400 Awarding 6% XP Bonus
LVL 4 Needs 750 Ally XP Totalling 1150 Awarding 8% XP Bonus
LVL 5 Needs 1000 Ally XP Totalling 2150 Awarding 10% XP Bonus
LVL 6 Needs 2500 Ally XP Totalling 4650 Awarding 11% XP Bonus
LVL 7 Needs 5000 Ally XP Totalling 9650 Awarding 12% XP Bonus
LVL 8 Needs 7500 Ally XP Totalling 17150 Awarding 13% XP Bonus
LVL 9 Needs 10000 Ally XP Totalling 27150 Awarding 14% XP Bonus
LVL10 Needs 20000 Ally XP Totalling 47150 Awarding 15% XP bonus
The new lvl 5 requirement for BFF's is now 2150 honor. It appears that some of the Ally XP rewarded for game modes has been altered as well. Guardian pays out 5 honor per round (half what it used to). Gridiron now pays out 10 honor per round (more than double pre patch). Will verify soon
BFF's can now be earned in 31 games of Gridiron, 144 games of guardian or 136 games of dodgeball. Given the changes Gridiron is the clear choice as BFF can now be earned in a single sitting.
Anyone who earned this before Operation 4 will now be Level 8 or 9 Allies which comes with a nice XP boost. A lobby with your BFF's can net you an additional 52%-56% additional XP. Let's hope they never release an "Ultimate BFF" achievement on a future update. The idea of having to earn an extra 21000 Ally XP is depressing.
The original solution will be kept available as a testament to how difficult this achievement once was.
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Allies are people on your friends list who who've played Gears 5 with. As the description states "four Level 5 Allies". This plus yourself is 5 people. All 5 people need to be friends and level 5 allies with each other to get BFFs to unlock.
This is the Ally XP requirement as of operation 2.
LVL 1 125 honor +2 % xp
LVL 2 500 honor +4 % xp
LVL 3 3000 honor +6 % xp
LVL 4 7500 honor +8 % xp
LVL 5 15000 honor +10 % xp
Total honor required 26125
Each level of Allies is accompanied by a 2% xp boost per person. A match where all your teammates are LVL 1 allies with you will result in an 8% bonus xp per match, LVL 2 is a 16% bonus and so on
*************************IMPORTANT******************************
You must choose a committed group for this. One person stops playing, everyone must start over. I can't stress this enough. It has to be one for all and all for one. Otherwise don't even attempt it. Its a waste of time. Special thanks to my LVL 5 Allies. xEaglEXAssassin, spxyu02, Maxiimum Chaos, EaglesFan520. Thank you for being good, honorable people.
Complete a Campaign chapter = 10 honor
Win a Versus Multiplayer round = 8 - 10 honor
Complete one chapter of Escape = 10 honor
Complete one wave of Horde = 3 honor
There is some debate over the fastest method.
Co-op Vs AI on Beginner leaves little to chance and in my opinion is the most efficient. Unfortunately, since Operation 3 was released the AI is more aggressive and doesn't group together. Taking them all out on Icebound with a single Dropshot will not happen anymore.
Dodgeball yields 32 honor per 4 round match averaging 450 - 480 honor/hour. This was how we started during operation 2. It was the highest honor value, however the extra round resulted in an average match over 4 minutes.
Guardian yields 30 honor per 3 round match averaging 480-510 honor/hour. As it is 1 fewer round we averaged under 4 minutes per match.
Either way it is a 55 hour grind minimum (about 880 matches). Realistically, 80 hours due to lag outs, dropped games, longer rounds, tardy allies, etc. Thus the importance of a committed group. It will take time to figure out the patterns of the AI on each map, As you do the matches per hour will increase and hopefully minimize the hours required.
This will take days if not weeks of cooperative playing. Always play together as a group. If there is a day where one person cant make it, skip the session. Grinding honor with less than the full squad will not get you any closer to the acheivement as everyone needs to be LVL 5 with each other for BFFs to unlock.
Best of luck.