Are you using shiki type 1 or 2 for Kuga?
I would recommend using the second and positioning your shiki return point near the top middle of the playfield. This will kill some waves quicker for safety, though netting less x8 action. For the boss of 4-1 and 4-2 in particular it is better to use their body as scratch points for the x8 normal shot (mostly at the start of the attacks). It's much harder to do consistently, but once learned it will be a quicker boss destruct and less time having to deal with the wicked attack patterns. Use shiki attack mainly for the final sliver of life on the different boss appendages -- more x8 coin action (note: there's no life guage to easily see this, you'll need to learn the timing without it).
There's more to it, though it's easier for me to do than explain. Hope this helps some anyway.