How can you shrink a Hill Giant?

Just use Psionics. Compression :: d20srd.org

It's personal only and has 1/10 th the duration, so it's a lot more limited than reduce person, but works on any creature. Even if not a psionic character, you can purchase tattoos of compression. Keep in mind you pay double* to get a personal targeted power as a tattoo. Psionic Tattoos :: d20srd.org

*Strange...my DM pointed that rule out to me a few months ago, but the SRD does not seem to contain it... Perhaps it was a houserule.

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