Up until Chapter 7, provided you bought the Expansion Pass, the best way to level up a Blade’s Trust is to buy Noponchos from Future Crafts in Leftheria and continuously drop them in a character’s Pouch before running out and buying more.
This is effective enough for achieving A-rank Trust, which unlocks all their Skill Rings, but the +21 to Trust is abysmally inefficient for increasing Trust to S+. Obtaining an S1 or higher rank is purely optional, but doing so will bestow damage multipliers for Specials, increase the Auto-Attack, and decrease Blade Cooldown Time.
The most effective method for increasing Trust is through the DLC item, Love Source. The New Quests Pack 3 will give a player ten Love Sources right off the bat. A single Love Source will increase a Blade’s Trust by 3,000. The Final Item Distribution List gives you ten Love Sources.
The catch with the Love Source is that making them is their own grind. First off, you can’t start the side quest to make them until you reach Chapter 7. Second off, the main ingredient for the Love Source, Flutterheart Grass, has an inventory cap of one. Only one will spawn in at a time, and you can only hold one at a time. Third off, you need four other Collectibles to brew the Love Source: Plumage Peach x2, Sumpkin x2, Dawn Hydrangea, and Melodious Melon. Below is the step-by-step process I used for grinding up Love Sources and the required Collectibles.
The Prep Step: Mute your TV or Switch and listen to some music or a podcast. You’ll be at this for a while, and if you leave your volume on, you’ll have to listen to the same one-liners, grunts of pain, and ten seconds of level music every time you fast-travel somewhere. This’ll be tedious enough as it is, so you might as well mitigate your descent into madness by slapping duct tape over everybody’s mouths.
You’ll also want six different Blades spread across your party: three with Agronomy and three with Botany. These are for maximizing the number of Collectibles you can get from a single spot. The Blades I used were Poppi QT, Kassandra, Crossette, Floren, Azami, and Dromarch. All had maxed out Field Skills. In New Game Plus, Akhos has Agronomy, and Cressidus has Botany. Common Blades work as well.
Also helpful is having the largest possible lifestyle item capacity. Ten doesn’t cut it for any segment of the game, so if you haven’t been scooping up deeds like you’re an eccentric tycoon, buy the deeds from the Nopox Hobby Shop at the Argentum Trade Guild and Praximo Cosmetics and Placks Patisserie in Theosoir. You’ll need to up the Development Level and complete the appropriate Merc Missions for these areas to get all the shops’ items to appear so you can buy them up and convince the owners to sell you the deeds. This’ll buff your lifestyle inventory limit to 40. These are the Merc Missions you need to complete and when you obtain them:
An easy way to increase the Development Level of a country (requires at least one country to be at 5 Stars) is to purchase Gold Cylinders from the 5-Star area for 5,000 gold and sell them at a lower-ranked area for 5,000 gold. Repeat as necessary.
One final thing is some throwaway Pouch Items. The Love Source lasts one hour, but if you’re planning on capping off the Trust of a decent number of Blades or all of them, you don’t want a Pouch automatically refilling and wasting a Love Source while you’re on one of the collection steps. If you just keep Blades equipped who need the boost juice, you can probably streamline this process by a few percentage points. But I’m not big-brained enough to keep track of that, so after I’m done with the final step, I’ll throw in a Common dessert or stable food or something to reset the Pouch.
The Flutterheart Grass Step: This is located at the Sealed Quarry in Tantal. The most efficient way to get to it is to fast-travel to the Airborne Throne, then jump down to the cliff with the Airflow. I’ll run in one small circle to give my health a second to heal up, then drop straight onto the ice slope pointed at the Sealed Quarry. Hold up on the left thumbstick the entire leap to maximize your distance and reduce the climbing on the vines you land on, then the Flutterheart Grass will be about ten peds to your 10:00.
The Melodious Melon Step: Right after traveling to the Goldmouth Flight Deck, I’ll run over to the garden with the Agronomy Collection Point, to the right of where you load in. I do this every time, since it supplies Melodious Melons, which are of Legendary status. After that, I run inside the entrance I was standing in when I arrived and go up to the Fiercest Fire to make my Love Source. If I don’t need any materials, I return to the Flutterheart Grass Step and repeat.
The Sumpkin & Dawn Hydrangea Step: This is a two-for-one, and you’ve got two options as well. The one I use most of the time, because it’s easier to fast-travel to repeatedly, is the Clear Sky Break on the Upper Level – Right of Gormott. Behind you will be the Collection Point.
Your other option is to go to the Torigoth Flower Patch, which is due south of the Clear Sky Break. This yields more items, but its Collection Points put you in the Torigoth map. So a few extra clicks are required to back out of that tab and fast-travel back to the Torigoth Flower Patch, unless you jog back to where you spawned in.
The Moment We’ve Been Working Toward: Once you got all the Love Sources you could need—or can fit in your inventory—it’s the moment of truth. Set up the Blades you want trusting you, and spam Love Sources ’til they’d all conk their heads together in a Trust Fall. The effect spreads across every Blade currently equipped to a Driver, and you can get the most efficiency out of your spamming in New Game Plus, when Rex can both equip every Blade and deequip Pyra and Mythra.
That’s it. The cap on Love Sources you can hold is 40, once you have the deeds from the above three shops, so after using up all of those, if you still need Trust increased on any Blades, return to the Flutterheard Grass Step and restart from there. Like any form of grinding, it’s less about challenge and more about weathering the tedium. Trust carries over into New Game Plus as well, so any grinding done in your original save file won’t go to waste.