I just take skills that would be useful for the statline and type of unit, the idea is to get skills that will make them busted so I can beat all the chambers, I normally play on a daily pace, when there are like 5 days left i do the push to finish all floors then if I were to forma a unit I would use all the torches on that unit on the first chamber to polish the skillset to get the skills I would consider optimal for a build and that are 5 star locked or seasonal locked, for example for Dozla I couldnt get a far save skill but I was able to get a menace skill which is premium and helps him no matter the build.
In my opinion grab all the premium skills that the unit can make use of for different builds, in case you cant get the optimal skill for the build you hope for.