I don't really know any solid combos with Reisen at the moment, but this is what I do currently.
With level 4 Mind Explosion (default 236):
66A 236B hjc9 j.6A
or 66A 236B hjc9 j.2A for roughly 2.1k. The great thing about this combo is that it works anywhere even with counter hit.
I tend to run her 5-cost spell card regardless of deck build since it connects quite easily on a j.2A. Hit (or counter hit) with j.2A and use Lunatic Red Eyes immediately after for easy damage.
Random Youmu Derp Combo of the Day:
(Requires Monsoon, Phosphoric Slash, knowing how to TK a j.2A, and/or a falling, airborne, (and possibly retarded) opponent in the corner.)236B hjc8 j.2A 236B hjc8 j.2A 236B hjc8 j.2A 236B hjc8 j.2A 236B "Slash of Eternity"
Deals roughly 4k damage. I don't remember if I was supposed to use the 5-card on the 4th or 5th 236B, but it's all the same really.
Edit:
WARNING: EXTREMELY CHEESY BNB FOR YOUMUFor the record, this was attempt #1548664684 or something. Yes, I realize I suck ass.
Edit2: While it is possible to perform this combo, without the TKing, at almost point-black range, TKing allows for a semi-long distance version of the combo that can be performed on a falling opponent in the corner (without Monsoon weather).
Edit3:
WARNING: EXTREMELY CHEESY BNB FOR YOUMU THAT DOES NOT REQUIRE MONSOON WEATHER.