A quick way I know is to use the Demoman to powertrain Dan until you have at least charm and use boost to get to at least transform, Val taking the hit of only being used for her freeze up until stoneskin. Demoman is easy to handle and if you know what you're doing you can't die to him. Rest whenever you feel like it but generally it takes me casting suppression I hit about 71% energy to get to charm. Rest and Boost up, Val should be either freezing or resting during all this. Then focus on your own Heal until the aforementioned Transform and then use Val to kill Demoman. From this you should have: Heal, Grapple, Transform, Suppression, Confusion, and Charm on Dan and Freeze, Sauna, and Melt on Val. If you'd rather not deal with the combat for that long, when you get home after dealing with Demoman, just spam heal on Val from home. With these you should be able to capture Val if you haven't already, do some of the special captures for Sarah and Julia, though I'd hold off on capturing Sarah for now (see below).

From there go through the base story, make sure to get the heart slime capture request after passing floor 4. Get to floor 5 and find the note for Dr. tiff's class, attend it with whoever you want, then backtrack to do the heart slime quest, after doing what you want with Julia. The heart slime is a lot like Demoman, use it to max out the conversion line until flirt for Dan (or get to trip and shrink if you haven't been spamming heal in your room and charm on enemies), and use this opportunity to max out Julia's main skill line. Heart slime isn't going to hurt much and if you have Val in your team alongside Julia you have a secondary healer to max out a spell line with. From this you should have: conversion, stoneskin, and flirt (maybe trip like I mentioned before) for Dan, which is pretty all the spells you need to do most of the stories for Dan and all at pretty low levels, like not even lvl 3 yet. With this you should have nearly every means outside the mysterious book for plot lines.

You can then backtrack and get Rack's class note for the physical spells for the other party members, then go to floor 7 for Drape's spell line (which can be sprinkled into average combat or focused on for monster captures) and floor 8 for Jane's spell line for party members. Needless to say these aren't exactly critical for story lines

Continue down Sarah's home plot line and capture Sarah's mom at your own pace if you want possession for Dan for the extra fun times

If you want the technical aspect of it all the newest spell of line grants more 'experience' than the last. Heal may be enough to get you to transform easily enough but transform will get you to trip a whole lot faster and always use the spell you just learned on Julia's main spell line to advance to the next one. When the wiki was up, it used to have the actual point value for each spell, but hopefully this small guide will help in it's absence.