Whoa, I've Just completed the 2 missions the two 10 millions tour.. You should try to keep all of the combos, especially those of the third and fourth level. Don't worry if you die, the combo can be maintained (if you are fast) during the respawn. Try to remember where the waves comes from and manatian the combo especially between two worlds.
At the end of the mission (Tour at the true difficulty), when I kept all possible combos, my final score was 11,5 M, without arkus. So, if you lost one combo (but only one) in the first or second world, maybe you can still do it eventually using 3 Points arlus at the end of a mission.
Hope it helps.. And sorry for my English