Don't kill stuff. Stall stall stall. A ton of points exist on the map, not in the plain enemies. Kill mid basses and basses, and thin the herd, but try to keep 1 enemy from each phase alive to slow the progression of enemies.
The diamond shaped enemies that explode bullets at you can be kited for quite a while, and I take advantage of this also, pushing enemy spawns back further.
Doing the above makes getting to the green arrows worth many points much easier, as well as eventually Cleopatra, if you can keep it up.