[Saiko Kila]

You can change the units you get, just use other summon spell instead, or whatever which uses the same sequence of pseudorandom numbers. For example:
1. save
2. use Call Colossus
3. if you don't get what you want then load
4. use, say, Call of death
5. use Call Colossus
6. if youd don't get what you want then load
7. use other combination of spell than before, like Song of the Wind (once), or two times Call of Death
8. use Call Colossus
9. if you don't get what you want, just load, change location - go to crypt or mine, or another mine, and repeat

Oh, one more thing. If you change location and come back (like, from grand canal to abyss and then to grand canal again) one of numbers from sequence is dropped and you get the next unit in queue, so you can actually save other scrolls and just keep changing locations instead of spending obsolete scroll.