avr |
Most of the levels on one side will be taken up by whatever means of becoming a gargantuan tree you use (if it's Awaken on a tree to create a gargantuan animated object then 9) so there's one main class to consider and maybe 1 level in another.
If you want to be a giant siege engine lobbing rocks then Stone Oracle 10 is your answer. For one level on the other side take whatever full BAB class meets your fancy.
If you want to shoot giant cannons then you want to start with a gun and to be able to enchant it - you'll never find another. Myrmidiarch Magus 10 // Tree 9 / Spellslinger Wizard 1 should work.