Until the release of the Byleth amiibo, there will be no official expert opinion on Byleth amiibo training. However, I’ve written a Smashboards post elaborating on their training, so this is the relevant excerpt from that post.
How to Train the Byleth amiibo
Raid Boss Byleth has only one unique playstyle option that has been labbed out thus far:
Super Armor, which ignores knockback below 132 KB units, will create a walking tank of a Byleth amiibo. You can only give Super Armor to an amiibo with a Support Spirit, so you’ll have to track down a Gold Mario spirit. In addition, run a -400 Attack, 4600 Defense loadout on Byleth. You can achieve this by putting on Support Spirits first, and then using the Absolutely Safe Capsule spirit.
For the record, this amiibo loadout is nearly unbeatable except by using high base-knockback projectiles. Camping out this Raid Boss with Charge Shot is a useful strategy if one can stay away long enough to charge the move.
Competitive Spirits Byleth is not set in stone yet, but it will definitely feature Weapon Attack Up.
Terry and Byleth are considered kin in the competitive amiibo scene. They’re both expected to be absolute monsters of an amiibo, and both have multiple tricks that will likely make them nearly unbeatable against other amiibo opponents. Byleth’s main trick is his Side Special, Areadbhar. Due to the way amiibo AI are programmed, amiibo will use an attack the very frame that they can connect with it. Fortunately for Byleth, the tipper hitbox on Side Special is enormous, resulting in a Byleth that can consistently hit the tipper. It’s not unusual for Byleth amiibo to KO their opponents at 70% or lower.
Thus, it’s optimal to train a very defensive Byleth amiibo. We’ll want a Byleth that only walks (running would only get the opponent inside of Areadbhar’s tipper), and almost exclusively uses Side Special. There is one exception to this rule: we want Byleth to use Down Smash when an opponent is approaching the ledge, or when they’re within Down Smash range. Be sure to use Down Smash periodically against your Byleth, as it’s their best point-blank defense option.
Avoid using anything on your Byleth amiibo besides Side Special and Down Smash. The Byleth amiibo can cover a massive chunk of the stage at any point in time using Areadbhar, and any time spent using other moves (even Failnaught) is time where Byleth is potentially vulnerable.