Every Act has 2 different Trial-Boss paths (I refer to them as “Left” and “Right” paths, although in the Seals display the Act 3 seals are in reverse order for some reason). Every unit you use on the Boss map earns the seal for that specific Boss.
The rest is SPOILERS so be warned.
IMO It really isn’t worth it to spoil yourself, figuring this stuff out for yourself with some trial and error is not hard and is very fun.
SPOILERS for different Act Bosses
There is an Act 4 Trial-Boss with its own master seal after completing a run and answering “Who” (I did this by writing down every name of every character and answering all of them until I found the right one, but it’s relatively simple in hindsight)
Acts 1-3 all have a Third “Middle” Boss path:
Act 1 Requires having the Ferry Ticket (Using Support to store it from an earlier run) at the start of your run and taking the Ferry
After Beating Act 1 Middle, back at Sortie talking to the Skills Shop lady will get you a Theatre Ticket (not an actual Item, you just have it now permanently afaik) to access the secret path of Act 2 from the first floor of the Act like with the ferryman. From here, you have to visit event nodes to view 3 cutscenes during Act 2 (does not have to be in the same run) to unlock the Middle Boss path, otherwise only the Left & Right paths will be available like normal)
After beating Act 2 Middle, there is a similar encounter at the start of Act 3, but I haven’t done this yet (literally just beat the Act 2 Middle Boss like 3 hours ago, lol)