As best, I can tell, Basil could "lift Mari's body by a rope into a tree at the age of 12", "take hits from Sunny", & is a "physically average high school student with a love of gardening".
What does Berdly have for combat experience? He lacks his Halberd, & I'm unsure if he did more than stand in place in battle firing off attacks. Sure he can dodge, but when he doesn't do so normally, it's unclear if Mr. Bookworm Bird Nerd Himself would do so very skillfully.
None of his Light World statistics have any justification, other than his AP stating that he appears to be an ordinary teenager.
Maybe he has hollow bones that are vulnerable to bludgeoning (Like punching.) like a regular bird, or maybe he doesn't because monsters in Deltarune might be made of magic like those in Undertale are? Or not, since Deltarune is supposed to be somewhat different?
His profile gives him Flight (No justification, though. Because bird I guess? & how well does/would he fly anyway?) so that'd be an option. ....If he had any range.
Stamina-wise, Basil is "Likely Average" vs Berdly's "Average", both somewhat understandably unjustified.
Mari was a 15 year old girl, & one source says an average 15 year old girl weighs about
52.16 kg. But to Basil's discredit, he lifted into her a tree using a rope; Using ropes to lift things makes lifting them easier because it redistributes the weight; The rope handles some of the weight & also changes how the object can be lifted, since, with a pulley, for example, you can, pull downwards using your body's whole weight or such.
Average Human Lifting Strength ranges from 50 to 80 kg, & what Basil achieved with the help of a rope is barely above that baseline. I didn't see EXACTLY how he used the rope to move Mari's body into a tree, so maybe it's less unimpressive in the scene, wherever that is; If anyone has a link to its official depiction, if any, I'd appreciate it.
Still if he struggled at all....