There's one big trick you can use to set yourself up for an easier grind. At the end of each floor, before using the key to advance to the next one, in addition to saving just before the door, run back to the previous save point and interact with it, even if you don't save again. This will ensure that the next time you loop to the start of that floor, your party will be as strong as they were at the end of it.

You'll also definitely want Memory of Family (double XP for your party members) equipped as soon as it's available, which means you have to have completed "In this moment..." in Act 3. Once you get Memory of Family, leave it equipped as much as you possibly can, until "OP" is completed. Every point of XP you get just playing the game is a point you don't have to grind for later.

It also helps if you have had the conversation with Loop that unlocks the ability to loop anywhere in the House at any time (related to the "Warning! Sharp!" achievement), because that will allow you to level Siffrin by looping after fighting the King (which gives 10k XP).

While you can technically do "OP" at any time, you shouldn't worry about starting the grind to maximum level (99 for Siffrin and 75 for everyone else) until you're at the end of Act 4. That way, you will have gained as much XP as possible just by naturally playing the game, minimizing how much time you need to spend on the really tedious grindy bits.

When I started, my party was about 60 and Siffrin was I think in the low 90s, and it took me about three hours to get everyone to maximum. So mentally prepare yourself for an extremely tedious time. (If nothing else, it'll help you get into Sif's mental space!)

So! You're at the end of Act 4, and you have Memory of Family equipped. Get ready for what is most likely several hours of tedium! It really helps if you have something else to do while you play, like a podcast or audiobook to listen to, or a TV show to watch (rewatches are good, because you'll need to glance at the game intermittently).

To start, you'll want to loop straight to unlocked Floor 3. This will ensure you're starting at the highest possible level. Next, repeat the following over and over (and over and over and over) until Isa and Odile are level 74 (Mira will always level up later, which is intentional):

1. Kill both Sadnesses in the room with the exit door.
2. Whatever side you ended up on, exit into that corridor.
3. Kill the first Sadness in that corridor.
4. Go back into the room with the exit door.
5. Every so often, save your progress at both the entrance to and exit from Floor 3.

So, for example: enter the exit door room; kill the left Sadness; kill the right Sadness; go into the right corridor and kill that Sadness; come back into the exit door room; kill the right Sadness; kill the left Sadness; go into the left corridor and kill that Sadness; come back into the exit door room; kill the left Sadness; kill the right Sadness... Ad infinitum ad nauseam, as my Latin teacher used to say.

At this point in the game, you should be able to one-hit most normal Sadnesses with Siffrin's "Just attack" craft skill. The one exception is Désespoir (the big hands), which takes a couple more hits but also gives the most XP of any regular Sadness (1k). For Désespoir: (1) Use "Just attack"; (2) if Mirabelle is second, use her "Jolly Round Rondo" (or her default attack); (3) use Isa's "YOUR TURN!!!" to pass back to Sif; (4) use "Buy One Get One Three."

You will kill these same four Sadnesses until part of your party hits 74. Be absolutely sure to save your party's progress at this point by interacting with the save point just before the exit door. (Optionally, you can also go back to save your progress at the save point before the entrance. It's a good idea to have a spawn point there with your party at 74, just in case something goes wrong and you need to try again.)

Next, you will repeat the following until Siffrin is level 99:

1. Unlock the door and beat the Floor 3 boss for 7k XP
2a. If you don't have the item that lets you loop anytime, use tears to loop back now to unlocked Floor 3.
2b. If you do have the item that lets you loop anytime, proceed and defeat the King for 10k XP (fast forward/zone out whenever possible to save time), then use your dagger to loop back to unlocked Floor 3. (Note that King XP only shows up at the start of the next fight!)

You can equip whatever memory you want while grinding Sif the rest of the way to 99, since none of your party's XP will be saved.

If you're only fighting the Floor 3 boss, you could alternatively fight the Floor 2 boss pair, since they together also give 7k XP. So it's just a matter of what's faster or more fun for you. (Or you could switch it up between the two.)

If you can't loop back anytime but want the option, try talking to Loop, and they may offer it to you. (You need to have looped with tears about 20 times before the conversation becomes available.)

If you find it takes you a long time to beat the King, you can just level up Siffrin with the floor boss, but the King does give about 40% more XP, so if you can fight him in less than one and a half times as much time as the floor boss, do both. Don't worry about setting yourself up to use the KnifeKey or the bomb; if Siffrin is so low level that they make the fight go noticeably faster, you should probably skip it anyway.

Once Siffrin is 99 and Isa and Odile are 74, loop back to unlocked Floor 3, ensure you have Memory of Family equipped again if you swapped it out, and go back to your party leveling procedure:

1. Kill both Sadnesses in the room with the exit door.
2. Whatever side you ended up on, exit into that corridor.
3. Kill the first Sadness in that corridor.
4. Go back into the room with the exit door.

Repeat until everybody else is 75. Congrats! You should now have the "OP" achievement.

If you find that everybody is at the proper level but you don't have the achievement, some people have had success with the following:

1. Hit maximum level on everybody, but don't see the achievement pop.
2. Cry. Despair. Feel even more close to Siffrin.
3. Examine everyone's profile and their stats.
4. Fight a regular Sadness.
5. Hopefully see the achievement now???

h/t John Helldiver in the guide comments for this solution

1. Equip Memory of Family ASAP.
2. Get to the end of Act 4.
3. Loop to unlocked Floor 3.
4. Kill both Sadnesses in that room.
5. Whatever side you ended up on, exit into that corridor.
6. Kill the first Sadness in that corridor.
7. Go back into the exit door room.
8. Repeat 4–7 until Isa and Odile are 74. Once they are, save your progress at the save point just before the Floor 3 door.
9. Unlock the door and beat the Floor 3 boss.
10a. If you don't have the item that lets you loop anytime, loop back now to unlocked Floor 3.
10b. If you do have the item that lets you loop anytime, proceed and defeat the King, then loop back to unlocked Floor 3.
11. Repeat 9–10 until Siffrin is 99.
12. Go back to unlocked Floor 3 if you're not there, and repeat 4–7 until your entire party is 75.
13. If you're max level on everybody but don't have the achievement yet, look at everybody's profile and stats, then fight a regular Sadness.