What difficulty are you on? Some of her spells tend to change rather drastically from one difficulty to another, so the answer will hinge on that.
Assuming you're on normal:
Milky Way: Unfortunately, there's not really a simple trick to this. Focus your attention on the small stars coming from the sides - they're the real killer in this spell - and be prepared to move around quite a bit. Unfortunately, playing solo Reimu is a bit of a handicap here, because you're stuck shooting the familiars instead of Marisa herself, which prolongs the phase somewhat. Don't be afraid to bomb if necessary.
Master Spark: The strategy basically boils down to "wait for her to fire, then run like hell to a relatively star-free area and wait for the screen to stop shaking." Easier said than done, of course, especially since you're running back and forth through the star spam. On the bright side, Reimu's homing attacks work in your favor here. If you can bait her into firing the spark off to the side so that you can get under her for a few seconds and deal extra damage, that's great, but survival should be the main priority.
Earth Light Ray: The lasers are a distraction. As flashy as they are, they're not the primary threat. The aimed bullets are the real danger, so treat this as a streaming pattern. Follow the gaps in the lasers back and forth across the screen, and when you need to turn around, dash over a little further than normal to open a gap. Also, try to stay a bit above the bottom of the screen - if you're hugging the bottom edge, the familiars will eat a lot of your attacks.
Unfortunately, a lot of Marisa's patterns are very brute-force oriented compared to Reimu's more gimmicky attacks, so advice can only take you so far. Just remember that the flashiest, most eye-catching part of the attack is usually there to distract you from the real threat. Focus your attention on the more subtle dangers instead, spend a while playing spell practice, and don't be afraid to bomb if you're not sure you can dodge an incoming attack. IN has a nice long time window for deathbombs, but it's still better to bomb before you get hit rather than after.
