Byleth scales to around 170.9 Megatons
Metagross scales to around 790.31 Megatons
There's a little wiggle room because they might both be superior to those numbers, but that's about a 4.6x advantage for Metagross, so Metagross is always hitting pretty hard.
And supereffectiveness is only a 2x modifer, not a 4x, so Byleth's fire and sword (if you count it) won't be enough to put them on even playingfields. Plus that's only a boost to their damage, Byleth doesn't have any durability boosting abilities on their page I can find.
Metagross also definitely takes an intelligence advantage, plus a mobility advantage with the flight abilities and can even range spam if necessary thanks to a superior range if it feels threatened enough to try that. (Byleth does have a flying counter move though, so there's at least that)
Metagross can also: Lower Byleth's speed, increase it's own speed, use moves that have augmented speed, increase it's own defenses, reduce Byleth's special resistance, lock-on to Byleth to reduce dodging chance, and completely overpowers in Lifting Strength when using telekinesis. Plus Byleth doesn't seem to have a resistance to mind manipulation, fear manipulation, or the flinching effect, so they're susceptible to attacks that will leave them confused or mildly stunned.
Byleth's pretty smart, and gets a few tries at this battle, but they have no way to reduce the incoming damage, even their strongest attacks seem to fall short of even being Metagross' equal, and Metagross has mobility, AP, intelligence, lifting strength, and a variety of abilities Byleth doesn't have any resistances to. I'm not convinced Byleth can find a victory strategy before they run out of Divine Pulses, I give this to Metagross.