I was also confused on how to determine the save DC. The MM doesn't seem to be as clear as you just were on that.
Save DCs for monster's special attacks are pretty straightforward.
The standard DC is 10 plus half the monster's HD plus an ability score modifier - Charisma in this case, like for most supernatural attacks.
If it's a spell or Spell Like Ability use the spell's DC instead (10 plus spell level plus ability modifier).
I actually had two feats and chose Weapon Focus (razor) since he's known to slice people up. But it seemed like I had a ton of special abilities & qualities for a CR 3 NPC. So I figured I was balancing him out a bit by having only 1 feat even if the build granted two.
But if it all looks good and the 2nd feat is appropriate, then I'll do that.
Well under the rules as written you have to give it 2 feats, since monsters always get feats for their Hit Dice unless they're mindless.
If you want to nudge his power down a bit I'd suggest making the 2nd feat something of little use in combat, such most skill feats.
About Wooden Body & Immunity to Magic; do you mean that I should simply mention the damage from Warpwood spell in the Immunity to Magic quality and do without the "Wooden Body" quality? Are you also saying that it should be called something besides "Immunity to Magic"? I'm not sure if yer saying the problem is with the name of the quality (Immunity to Magic) since that name is used differently on other creatures, or if my description to the quality is not allowable.
Basically, if you followed the standard Immunity to Magic it'd look something like this:
Immunity to Magic (Ex): A Maligno is immune to any spell or spell-like ability that allows spell resistance. In addition, certain spells and effects function differently against the creature, as noted below.
A
warp wood spell cast on a Maligno inflicts 3d6 points of damage and slows it for 3 rounds.
If I were writing it I'd add some more wood spells, e.g.:
An ironwood spell spell cast on a Maligno gives it a +5 enhancement bonus to its natural armour for a duration of 2d6+12 minutes.
A repel wood spell pushes a Maligno away as if it were a wooden object.
A warp wood spell cast on a Maligno inflicts 3d6 points of damage and slows it for 3 rounds.
A wood shape spell cast on a Maligno can be used to inflict 1d8 hit points of damage or repair 1d8 points of damage (caster's choice).
The Maligno gets no saving throw against any of these effects.
Of course, most of those spells are too high level for your party to cast on Maligno, but I like them just as a thought-experiment.