The path is laid out below; I change from Red pen to Blue pen in the Boudoir. The entire red pen line comes first. The blue pen line begins after you kill the succubus. I appreciate that's confusing, but I am awful at paint.
Map clarifications:
The map is sort of unclear due to the different pen overlap in one spot - the first time you come to the observatory, unlock the door to the right, but double-back and don't go through (as with several other doors you see).
Also, after you go through the N bath house door (your first visit to the bath house), head into the next corridor and go N, clearing both of the 'under' rooms. The second one has a freezer with a water elemental and 8 billion boxes. After looting that room, head back S and resume your path to the observatory.
If you're desperate for time, but have many crests of all varieties, consider killing Dr Rushmoor and getting the final doors before heading up to Crystal the Giant. Conversely, if you have plenty of time, but lack crests, settle for just killing Crystal for the score, then go try and farm crests through kills, but prepare for disappointment. If you have plenty of both (you should), I find it more convenient to kill Crystal first as shown on the map.
Other Things to Note:
This is just the broad outline.
Get every side room and crest. I haven't, e.g. in the servants quarters before the first door drawn a billion crossing over lines showing you getting the 3 downstairs to the R side rooms, and all the upstairs side rooms - which you should do.
Smash every breakable you see. Kill what's convenient, occasionally grouping up trash for a good ol' fashioned mass beatdown. AOE fire attacks to blow up crates are a plus. As is pick locks / knock.
Only other spoiler I'll include is to make sure you go 'up' in the Banner room. You don't need the jump power-up, but you do need a reasonable jump score, and Featherfall. Up to 9 crests can spawn up there. Get the grand chandelier in the centre last.
With some luck (there's invariably some) you'll 5 star as I have with this path. The luck shouldn't kick in until literally the last door or so. Prior to that, you will almost certainly have enough of all types. If you end up facing 8 wolves when you have 7 of everything else and exactly 1 wolf. Well hey. Random generation fun. That said you pickup literally every guaranteed drop save the rubbish in and around the bathroom, which never seems to have a crest for me regardless.
Other varations:
Picture Portals - do the exact same thing. You have less time though, so hurry. Also consider not going through the Secret Door to the Banner Room, and instead going through the Restroom tunnel. This is solely so the stupid kobold doesn't get stuck as he's want to do on the stairs down to the spider tunnel unless you wait patiently at the bottom.
Moving Targets - This one is a bigger pain to me. I'm not sure of the trick... were I in a group I'd just designate a kill squad, but solo I think you just have to cop potentially doubling-back a lot; or changing the route dynamically to go directly to bosses. Either way, blerg.