Alright. Don’t know what’s in-character for Darcia or not, either way let’s compare their strengths and weaknesses:
Tangrowth scales to Tyranitar, making him 326 Megatons. Darcia scales to Vandalieu, who has a mid-end of about 5 Mountains of Energy, or about 131 Megatons. The calc goes higher, but this is the mid-end, and therefore I’ll assume that we don’t use the High-End unless told otherwise.
Therefore, Tangrowth has the AP Advantage.
Tangrowth is rated at High Intelligence for Combat, and Average for everything else. This is because Pokemon are practically bred to fight from the literal instant they hatch, and Tangrowth itself is a Predator.
Darcia is Above Average, High when using Goddess Descend, which gives 1000 years of intellect.
Albeit not major, Darcia has the Intellect advantage.
Tangrowth has standard Melee range, but can extend that and reaches tens of Kilometers with most attacks.
Darcia has standard melee range, but up to about a hundred Meters with their bow.
Tangrowth has a significant Range Advantage.
Tangrowth typically leads with ensnaring its opponents with its Vines, which it can do easily. Starting at a range of at least 4 Kilometers though, it’s likely it’ll attempt Ranged attacks first, especially with its better use of Special Attacks and wide coverage. Plant Pokemon also tend to spam their Status attacks, and 14/20 of its moves are Status attacks or Special Attacks, increasing the likelihood. We already covered what these attacks do.
I don’t know Darcias starting move, but they have laughably inferior Range with their attacks. Darcia seems to have a lot of skill, good with both their Bow and Dagger, and able to change up their fighting style easily. Darcia can amp themselves up, summon a Beast to shred away at Tangrowth, or manipulate Water and Wind to blast at him. They can even change their bodily shape, and go Intangible, the latter of which is useless due to Tangrowth‘s NPI.
Overall, the best abilities I say goes to... Tangrowth. He simply has far more combat-applicable abilities, Status effects, and can decrease Darcias stat amps in response or even amp itself alongside that. Darcia has Regen, but so does Tangrowth, and both have Healing but Tangrowth has self-healing rather than Darcias normal healing. The massive range advantage doesn’t help either, and while Darcia has Air Manip to attack Tangrowth I’m not sure if they’d lead with it, in addition to all the Healing and Regen making it hard to do solid damage.
Tangrowth is rated at High Stamina, for good reason, able to hold its own through long, drawn out fights against multiple opponents, even as its weaker form.
Darcia is rated at Likely High Stamina due to the sheer amount of skills that increase it.
Without further info, I’d say both are pretty even in the Stamina department.
Over all, I’d say Tangrowth takes this. His Vine Regen protects his vines from just getting shredded, has access to several amounts of self-healing, Stat Amps, Stat Debuffs, and a hefty Range advantage. Darcia just has more Stat Amps than Tangrowth and the ability to exploit its weakness of Air Manip. Tangrowth has a solid AP advantage (at least 2x), and while it May take a bit moves like Leech Seed, Constrict or Confusion can Incap Darcia via its severe Lifting Strength advantages or Confusing Darcia to oblivion once it learns it’ll be difficult to kill Darcia for good. Iirc you also don’t start with ‘Possibly’ abilities now, meaning Darcias Regen is just Mid, which can be overcome with enough force.
Quick question- how good’s Darcia’s Status Effect Resistance? Tangrowth has a potency of at least 108 last time I checked, for being able to affect Spiritomb who is made of 108 Souls. If Darcias doesn’t have a feat, its just baseline resistance, further solidifying Tangrowth’s win.
tl;dr Tangrowth has most advantages and lots of ways to incap + Darcia may not start with possibly Regen now meaning their Regen is weaker to the point where it can be overcome. (further research shows that was wrong, the agreed upon answer was not to restrict Possibly’s, my bad). Tangrowth may be able to overcome Darcias resistance though, with further evidence provided.