Fuga: Melodies of Steel 2

This achievement can't be done during your first playthrough, and you should really not worry about it while playing first time.

In general, I would of course invite you to explore your best team-ups during the first chapters, but in my opinion from let's say chapter 7 onward you should have your optimal team with 3 couples having a level 10 affinity to fight bosses or other difficult encounters.

To give you perspective, I constantly switched couples as soon as one reached 10 affinity, constantly fighting with the experience and affinity bonus food from the messroom (which in general I would still advice you do for the experience more than the affinity) during my first playthrough and I still ended with around 12 couples at 10 and a bunch at 5. It's not worth it considering the real trick for this achievement is the following:

In New game plus, in any farm shop, for the price of 100.000 coins (or 80.000 if you wait for a chapter with coupon) you will be able to buy a "Golden PawPaw" that will double affinity gains.

To give perspective, when I was just finishing this last achievement, each intermission with 20AP I was easily doing almost 2 couples per intermission (supposing you will get correctly all the "chance to deepen your bond" opportunity every 2-3 AP talking to the same character. If you are not able to choose at least the option that gives 2 hearts you should be ashamed of yourself, it's pretty obvious). If you are running only for the achievement, and don't care about completing a second time the game to unlock all the files with Crusade and Jihl backstory, just choose the safe route each time, don't explore caves, and just rush intermission to intermission.

One last tip is to be smart about you AP investments. During the first playthrough, investing AP in talking is a complete waste. You are better of with the messroom food to increase gains of experience and affinity during battle and basically everything else.
After your characters ate on average at level 30, the only worse investment is putting to sleep characters for XP. 5APs is too much for 2500 or 3000XP. Also, in general scrap fishing is useless up to the point you will need ancient materials at the end game. Sell the materials you don't need and just buy whatever you need.
After you have the golden pawpaw, depending on the couple and the level of affinity you are currently at, you will generally do at least half a bar each time you will talk. In the deepen your bond opportunity, 1 full bar is almost guaranteed. More during the first levels. This means that if you are at level 9, and you are at three quarters of a bar, investing 1AP to finish is a complete waste. Just put the couple in the battlefield and you will complete it there.