1. Get to at least Chapter 8 in the game.
2. If you've already defeated Melusine, use Chapter Select to replay Chapter 8 or later. This will wipe out a lot of your progress like fast travel checkpoints, so don't save over your current save files with it. It would be preferable if you had an old save from before you defeated Melusine but after reaching Chapter 8 that you can load up instead; that's what I did.
3. If you're at Altissia or Chapter 15, go to a rest area and select Umbra, then use the canine's top option so he will take you back to past Lucis.
4. Fast travel or drive to Meldacio Hunter HQ and Start the O Partner, My Partner quest by talking to one of the hunters there.
5. Equip a Ribbon or at least an Earth Pendant on each of your party members so they don't get petrified.
6. Travel to the Melusine boss fight location around Vesperpool. Wait until night when she comes out. You may end up having to wait for her multiple times.
7. Engage Melusine in combat. Wait for her to use a jump and slam down attack on you. Follow the button prompts to block with
and parry/counterattack with
. If you did it right, you'll chuck a grenade at her and blow it up in her face, giving her Vunerable status. If you failed, she'll grab you and bitch-slap you a few times, so be careful not to miss the
.
8. Warp-strike Melusine while she's vunerable. This will initiate a Cross-Chain. As soon as the button prompts in the right side of the circle in the middle of the screen with the numbers inside it shows the
button, press it and the counter will go up. Do this until the counter reaches 10.
9. You need 10 cross-chains in the same save file/playthrough to get the trophy, but Melusine will only give the cross-chain prompt once per fight. Step 10 is the fastest, easiest, and only sure-fire way to earn it, but it's
really stupid, and an oversight on the devs' part that you have to do this.
10. Run out of Melusine's battle radius and repeat Steps 7-8 nine more times.
And that's it.
Credit to Rubedo_URTV_666 on https://gamefaqs.gamespot.com/boards/932981-final-fantasy-xv... for sharing this method so I could elaborate on it and share it with you.