After some progress in the story, these two options will become available to you on the main menu. Simply send some items to a friend and trade soldiers with someone to get two trophies. Note that you can only trade soldiers if they're the same rank, so you can't send your friend 50 S Rank Intel soldiers to give them a boost in the early stages of the game.
Versus Ops & Camaraderie
There's one trophy for playing all four modes of multiplayer with someone. These modes are Team Deathmatch, Capture Mission, Base Mission, and Deathmatch. This can easily be boosted with a friend since the current online community is as dead as The Boss. All four can be set into a playlist, just change the settings to the lowest values to make the games finish very quickly.
The other trophy for online play asks for you to get 20,000 points of Camaraderie with other players, which is a measure of your accomplishments and friendship as you complete missions. To grind for Camaraderie with others, try to get in as big a group as possible and play either Extra Ops 127 or 128. For Extra Ops 127, prepare yourselves for an S Rank (help others get the Infinity Bandana first if you're kind enough) and wear the Battle Dress w/Helmet. Have one person bring their best stun and sleep gas grenades, and have them first stun Gear REX, followed by about four sleep gas grenades. Then get another stun grenade ready, and repeat the process. While that player prevents Gear REX from moving and being any sort of hazard, the other 1-3 players will be shooting Gear REX with Hush Puppy rounds until he passes out. The fight can be done in about 4-5 minutes with three people and even faster with only two (but giving you less Camaraderie), giving you about 150+ Camaraderie per player you're playing with.
To give yourself even more Camaraderie, try to stay CO-OPs In with other players (get close to them to activate a yellow circle). The CO-OPs In boost can spike up your Camaraderie with anyone you were doing it with to about 450. Extra Ops 128 probably gives you even more Camaraderie; my group wasn't coordinated enough to finish it, resulting in us discovering that Extra Ops 127 was just fine and much easier. If everyone stays CO-OPs In for as much as they can, it should only take about 15 missions of Extra Ops 127 with four people to get your trophy.
If you want to idle boost this with someone or a second console, a method from S3M James on TrueAchievements shows that you can set up a Deathmatch in Versus Ops to whatever amount of time you feel like and set the game up to run until you don't feel like boosting anymore. Setting the ticket counter (score limit) to 1 makes the rounds end fast when you want the game to be over, and continuous rounds will start on their own. If you and your boosting partner want to boost 8 matches of 2 rounds of 99-minute deathmatches overnight, then the only thing stopping this from working is internet connection problems. You can have up to six people with this method, and the more people there are, the more Camaraderie everyone gets; it's a win-win situation.
Reached a total of 20,000 or more Camaraderie.
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