Not bad, but probably still insufficient: it still needs some kind of ranged combat capabilities.

A wild magic effect has a 35% chance of hitting anyhow, and a 2% chance to hit maximized. It has a 12% chance of not using up the spell slot. The rest of the chances are effectively wasted.

Blocking the acid arrow damage or healing it is easy from reflected arrows. Random target arrows will be very spread out (1200' range at L 20) -- so some peasants will die.

9*7 damage per cast, times .39, using up .88 spell slots.
24 damage per cast for .88 spell slots.
27 damage per spell slot used.

Overcoming fast healing 10 is relatively easy once a few arrows have hit hit.

Note that this isn't about acid arrow as much as it is about "any attack from a reasonable range that can actually harm the target can relatively rapidly kill it". You just have to do more than 45 damage/round (if doing physical damage) or more than 10 damage/round (if your damage bypasses the creature's immunities) by a significant margin.

This is stressed by the fact that approaching this creature is pretty stupid. I suspect that even an L 29 epic fighter would be schooled by the creature in direct combat.

Let's suppose a +10 lance with +6d6 bonus damage from a variety of sources. The L 29 epic fighter has 24 BaB. Let's try a charge+leap based build, with a truestrike buff to make him hit anyhow.

He's also enlarged 2 categories, and has powerful build, so he's using a G sized 2HS.

And 36 strength.

(4d6+72+13+10)*3+6d6

348 average damage per hit.

+40 to hit (0 BaB, 13 strength, 20 true strike, -3 size, 10 enchantment), so it relatively reliably hits.

Pounce from some source.

+40/+35/+30/+25
4+/9+/14+/19+ to hit.
1.9 hits on average

So, the charge build, on average, kills the creature in one round. Ok I'm wrong: it looks like epic melee cheese could do him in.

Now, what would it do attack-wise against an epic melee?

Slam: 68 damage
Full attack: 248 damage
+57 grapple check (assuming it uses only the tentacle).

A grapple monkey would have +24 BaB, +13 stat, +4 to 12 misc bonuses (feats, etc), +8 size, and might remove 6 points off of that grapple bonus.

+57 vs +51 -- a grapple-monkey it would not reliably avoid being grappled with a single tentacle. Non-grapple-monkies would very reliably get grappled on every hit, and never get loose.

That's 68 extra damage on the grapple, and nullifies one attacker.

Now, what are the expected HP of a L 29ish fighter-type?

(sorry about the metagame analysis -- but that is how I think about creature design!)