You can savescum the spawn of new hives, it happens at the moment you first enter the portal in the day, although I believe the spawn chance and number is related to how long it has been since you last visited that portal.It keeps happening. I picked that difficulty option that makes new portals spawn less often, which, coupled with the challenge level that does the same thing, should logically mean that Creation would have less chances to spawn new hives, and therefore limit her corruption growth, right? Well, apparently my earlier speculation is true and her corruption increases by a flat amount each day (by 2 I believe), regardless of whether new hives have been spawned or not. Now I have, at most, 4 active portals at any time (not counting unstable) with no new hives in them for days on end, while her corruption just keeps increasing.
Conclusion, until this is fixed (assuming it isn't intended, or if I didn't just shoot myself in the foot for severely limiting portal spawns) I imagine the only way to rescue Siena and snag that achievement is to do so on a very short run, otherwise her corruption will eventually outpace you.
sigh. *new game*
If you are playing as Evolution, picking all the "nice" options (let Siena powerup and talk with her after the fight) will reduce her corruption by 10, using the Cleansing Staff's active on her during the battle will reduce it by another 10. So you can reliably get -20 if you meet her in an artifact portal (also something you can savescum for in case you run into the Druid instead of Creation).
I think that for all the "purification" type of endings (either purifying Siena in the final battle or getting her as an ally if you're on the alternative ending route) you need her corruption to be under 75. So the best way of doing it in a long run (where she will very likely reach 100 corruption while you're minding your business) is to try to save a few portals with multiple hives that you can quickly reach (either with flying movement or previously setup relays) and do them in the same day where you do your last artifact portal before going for the final portal.
Keeping her corruption low while playing as the Druid on the other hand would be a massive pain in the ass since you can only do it by eliminating the hives, good thing there isn't an achievement for that.