Just to confirm that yeah, the reason you take off Wild Heart is cause Half Shift is only 1.5x stats vs. Human form, contrast to normal Shift which is 2x; its why Volug gets significantly better in Part 3. He can take off Wild Heart, and finally get full Transformation Benefits, and Olivi Grass (which you should always get your hands on some; I think Vika comes with some, so giving hers to Volug is ideal before Part 1 ends) lets him (and any Laguz really), maintain the shifted form for quite a while, so long as you use it at right intervals rather than attacking.
The thing Volug does is able to hold his own without help beyond some items (which he can hold on his own; what's he need those inventory slots for anyway?), and generally is damn hard to kill, and he's not failing on offense either. And all his worthwhile requirements are...things that don't really hurt other PCs.
No one else in the team is a Laguz, so Olivi Grass benefits him and him alone. Items can be bought in bulk generally, so giving him a few won't hurt. He requires no weapons, so stuff like "Should I give that Brave Sword to Zihark, Meg or Edward?" never kicks in. He's not relying on stuff like Beastfoe, cause he doesn't need it, doesn't need a healer...
Honestly, he's a lot more self maintaining than any other PC here. You COULD argue someone like Jill may be overall better on this map due to her being mobile, especially on this map, and having the durability to rush in for a bit, take a beating, heal up, etc...even ignoring my Jill Fanboyism, that's a bit more reasonable of an argument, since Jill is doing stuff no one else can do, and is being generally effective without the aid of others, nor is she relying on gimping OTHER PCs (only thing she may be using is the Brave Axe which only Nolan can use otherwise, and Nolan's better off sticking with Tarvos due to the defense boost on this map generally, since Nolan's taking a more defensive stance, so Brave Axe is better suited for Jill here overall.)
I just don't see Leonardo hype. Give Zihark that Beast Foe, for example, and hand him one of those good early swords like I believe there's an early Brave Sword or Silver Sword or hell, just a forged Steel Sword, and he'll be doing shit just as good as Leonardo or Nolan, and since he's primarily relying on Counters and I do not believe Vantage is available at this point for the Dawn Brigade (from what I recall, the scrolls are Innate on Mia, Innate on Naesala, and a drop by Ludveck), 1 hit vs. 2 Hits really doesn't matter beyond weapon conservation.
Lets look at all PCs in the Dawn Brigade and see their strengths, weaknesses, etc. for Chap 3-6, and evaluate just why Leonardo doesn't stand out much:
Micaiah: Healer, can attack from range thus avoid counters, but generally, she's acting as a healer here, which is useful since she's one of two, and generally the more durable of the two.
Sothe: Still has a level lead from before, the Micaiah A Support probably still around, and best yet, Beast Killers. They're probably still close to his strongest weapon, with the +20% Crit Rate, meaning he doesn't even need Beast Foe. Best yet, you can buy one in this chapter immediately, so any arguments about having them left over from Previous Chapters sort of dies (and saving them for later chapters...when they'/re important again, a 3rd one pops up IIRC, and after that, they're pretty much entirely replacable by Stilletto's which are roughly the same thing.)
Edward: Caladbolg w/ Beast Foe should let him kill things, and if he's promoted, he's got a steady Crit Rate or something! And some passable evasion...yeah, he's not really that good unless he's RNG Blessed and leveled up to speed, both factors I'm not willing to assume.
Leonardo: He's on trial, so not gonna discuss him. Basically, in short, its "Beast Foe + Crossbow = Yay!" but I'm trying to prove why that's not too special.
Nolan: Unlike Leonardo, he does NOT require Beast Foe to be reliable; he's likely got less level problems for a number of reasons, Tarvos gives him that durability push, and he's fast enough to avoid being doubled in general, possibly doubling Tigers too.
Laura: Your other Healer; that's literally all she's useful for as at least Micaiah had offense, though having a 2nd one that isn't taking up any one else's slot is welcome. This allows for far more flexibility, as it means Micaiah being forced to take offense for one turn if you need that enemy dead won't be QUITE as risky.
Aran: He's a steady tank, and honestly, that alone makes him useful here. If his Lance levels are high enough (which I wouldn't say is a given), he can wiled Tauroneo's Silver Lance for that strong weapon with Beast Foe combo, again proving that Leonardo is unspecial in that regard..
Meg: Aran variant; a bit worse since Javelins > Wind Edges for ranged, uncounterable options, Lances > Swords for power, and Aran has more move, but she can still be useful as another tank in the front lines, assuming she's leveled of course (which should be assumed for any character, barring a select few like Sothe and Zihark who have no excuse for being underleveled here <_<) Oh, Meg does lack that Silver Lance thing Aran has, but I believe a few decent swords are picked up in Part 1 like a Brave Sword or something.
Volug: we've covered this countless times, not repeating it.
Jill: More or less covered already, don't need to do it again.
Zihark: Fast enough that he MAY double Cats, and RNG proof at this point, combined with insured to have at least some decent level stock. He's pretty much a better Edward, as the level edge > Caladbolg (and he can use Brave Sword to offset the offensive disadvantage if you so want to play that card.)
Fiona: ...ok, she's got issues; Mounted unit in this water level is bad, and she's still probably trying to work away her underleveledness, her only real use is emergency rescues due to Savior.
I believe that more or less covers everyone. Honestly, most people seem to bring more to the table than Leonardo. Over half the cast has some strong weapon to take advantage of Beast Foe with, while people like Meg and Aran are formidable tanks that would lower the strain on your healers.
I dunno, Leonardo having a SPECIFIC SET UP that lets him have massive offense but does nothing to save his defenses makes him feel utterly un-special here. At best, it lets him attack and kill without working about counters, and really, that's not that major of an advantage cause Fog of War and the Water based aspect of this level makes it hard to really go on the offensive, you're better off luring enemies in and beating them defensively.