Guardian Spirit "build"

I gave my level 10 hunter the Summon Guardian Spirit feat and now I am thinking about which base creature to take and how to build it.

My first take:
A lightning mephit with the ill omen and call lightning SLAs.
Combined with its natural lightning bolt and shocking grasp as well as its lightning breath it has some potential unless the enemies are resistant/immune to electricity.

Which other base creatures would you suggest (fey or outsiders that can be taken as improved familiars)?
The template can be found here, below the imp guardian spirit.

Which feat would you give the guardian spirit for the 2 HD it gains when summoned via SNA4?

I'm not sure about the breath weapon but it should increase by two dice, one for the higher level and one because it is a standard action with instantanous effect.
How about the lightning damage on the claws, is that an ex or su ability? if so it should increase to 2d4.

No suggestions at all?

FYI, the lightning mephit is 3rd party material: Lightning Mephit from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Author Scott Greene.

In case this is not a problem, or if you pick a core-mephit, the breath damage should increase is you said, but the claws damage not: it's just a different type of damage, like 1d4 slashing and 1d4 piercing, so it doesn't increases.

For the feat, it depends on how you want to use this creature. If you want it to fight melee pick Power Attack or a Martial/Exotic Weapon proficiency feat for a weapon you give to it every time you summon it. If you want it to do instead AoE via its breath pick Ability Focus.

Thanks for the answer.
I realized its a third party monster and contemplated selecting a different one. Air might be nice as it has good SLAs, too.

Good to know about the claw damage.

As to how I want to use the creature: I mostly want to have it use its SLAs and breath just to increase my action economy. Instead of casting call lightning myself I can now summon my guardian spirit and have IT cast call lightning, use its breath or other SLAs.

Remember that the Mephit can cast his SLAs once per 24h, and it's always the same between multiple summons. So If you summon it 3 times in one day (spending 3 spell slots), it can cast Call Lightning just once, so it's not very convenient to use it instead of your spellcasting.

Just use it the same way you use any summoned monster. So meat shield and physical battlefield control. The breath helps with the control, and SLAs increase versatility.

I realized its a third party monster and contemplated selecting a different one.

I'm just going to throw in a suggestion for an aether elemental. Flight, blindsense, natural invisibility, a slew of immunities, and several handy telekinetic abilities make a very nice chassis to boost with Summon Guardian Spirit.