Are we talking about the first or second game?
The first mostly boils down to efficient use of kakusei. As a general rule, it’s better to save up a respectable number of gems before going into kakusei and then being patient with large hordes of enemies so that they can litter the screen with bullets that you can then cash in on (by killing them). Trying to play for score in various areas of the game can actually make it easier to survive since you’re staying in kakusei for longer periods of time.
Pushing the game into max rank by the end of the first level is actually recommended as it increases the max jewel release from enemies once they die (kill Seseri’s first form and then just go into OD and remain there and dodge until the 3 gradually increasing blue circles around your character are gone). Then switch back to normal and kill her. So long as you can refrain from getting hit, you’ll remain in max rank which grants you +15% gems on kills… another +10% comes from killing enemies when you’re in the upper third of the screen (both bonuses are calculated from an enemy’s base gem value… so once you have both active you’re looking at a bonus 25% per kill). Of course, once you get hit rank plummets back to normal.
Also, if you’re having trouble with bosses, there’s no shame in saving up some gems prior to the fight so you can slow them down during difficult patterns.
I think the stumbling block most people run into is Jakko’s final form. Here’s the thing, this attack only has two variations. The way to approach it is to stay in the center bottom of the screen (about 1 or 2 character heights from the bottom). The pattern interweaves from the left and right and the best starting point is between rows 2 and 3 (IIRC). The left side will either raise first or lower first… that’s it. Remain in the central gaps between these two bending streams for as long as possible before dipping down through the 2 streams into the large gap created below. You should be able to stay here for a good 8 seconds or so. Just be cognizant of the vertical bullets that will start raining down. The next dodge will either be to the left or right depending depending on which way the streams are moving. Beyond that, the most important thing to remember is NEVER HIT THE FINAL FORM WITH YOUR BOMB (Jakko actually heals life from being hit by your guard barrier)! If things get hairy and you need to tap the energy release to clear the screen temporarily, as soon as you hit the button, immediately move to the left edge of the screen! This way you can discharge the power barrier without any of it contacting the boss (as he/she mostly stays in the upper middle of the screen. Also, use kakusei sparingly during the fight (the boss takes damage at a much lower rate).
If you need help with the second, I’m afraid I can’t be as helpful. The second game is actually quite difficult and the scoring system is insanely deep. As before, getting into max rank by the end of the first stage is recommended (luckily it remains at max for the entire credit… so you don’t have to worry about it disappearing after you take a hit). Also, there’s no shame in abusing the tutorial glitch (it can just be annoying to wait for it). Beyond that, just remember that you collect way more gems from enemies than in the first game and they drain the meter at a much lower rate when killed. Zetsushikai Kakusei constitutes the game’s primary scoring… so essentially, save up some gems, make sure you have some gold and then hold down that kakusei button. In zetsu, be very patient with killing enemies, as destroying even a small zako will despawn-then-respawn the zetsu bullets. Get used to tapping the auto-fire button (as opposed to the normal shot button) since doing so will release a single salvo of shot which is better for targeting smaller enemies and not mass killing everything. Try to use sweeping motions, as in, kill an enemy which’ll cause the zetsu rounds to despawn and then move to the opposite side of the screen as the bullets start to respawn. This will then shift their trajectories to that side. Kill another enemy and then move back… wash, rinse, repeat. The other big trick is learning when it’s effective to hold down the kakusei button… as in, you have no gold but a load of gems. If you press and hold kakusei indefinitely, you’ll enter regular kakusei until you kill something. As you collect the gold, the game will shift you immediately into zetsu… so long as you haven’t let go of the kakusei button.
One final note, if you start feeling like the screen is getting too busy in zetsu, so long as you still have some gems left, switch back to normal mode and then trigger regular kakusei. Kill any one enemy and all the remaining zetsu rounds will clear permanently from the screen.
Aside from that, good luck.