And I would argue Sanctum Spell doesn't get you there either, as it (in this case) only allows the character to cast a 1st-level spell with a higher "effective" spell level while in a specific place (a site that can be no larger than 100 feet in diameter prior to the character's level reaching 6th).
IOW, I don't believe there's a way to "shortcut" the requirement to be able to cast 2nd-level arcane spells: the character has to actually be able to cast 2nd-level arcane spells, not have a highly conditional ability to cast a pseudo-heightened* 1st-level arcane spell.
* It's telling that with Heighten Spell the spell is prepared in a slot of the heightened level (a 1st-level duskblade has access only to 0th- and 1st-level spell slots), whereas a Sanctum Spell is prepared in a slot of its "correct" level.
I read the requirement "ability to cast 2nd-level arcane spells" as pretty much mandating the character have 2nd-level spell slots. Wizard or Beguiler 4/Druid 3 looks like the fastest route to me.
JMHO. HTH. HAND.
P.S. Is there something wrong with going straight druid? You get wild shape at 5th level and can pick Natural Spell at 6th, by which time you're also rocking 3rd-level druid spells.
P.P.S. You also need the Trackless Step class feature. Bamboo spirit folk receive Trackless Step as a racial feature. I would argue that a racial feature does not meet the requirement. (The Dragon 318 OA update didn't suggest any changes to spirit folk.) The guide below mentions that a player might be able to convince a DM to allow it, but I wouldn't.
P.P.P.S. This guide (
http://community.wizards.com/go/thread/view/75882/19868490/The_Arcane_Hierophant_Examined) examines various builds that can enter Arcane Hierophant. None are lower in level than 7th (gaining AH as 8th character level). I would be very suspicious of a build that can enter at 5th (gaining AH at 6th character level). The guide author declines to consider certain available "workarounds" for the spellcasting requirement as they are so controversial.