Build guides for Guardian Spirit? If not, any help?

Hmm.

Silvanshees are not fond of open combat, even against demons, devils, or other fiendish threats, and they’re likely to run away if confronted. When they must fight, they prefer greater numbers and the element of surprise, using their magic to temporarily overcome their physical weaknesses, and melting away into mist if the battle turns against them.

Basically you're herding a cat here.

Probably not an issue if even heightened mount/alter summoned monster cheese is allowed, but 'When you summon your guardian spirit, the duration of the spell you use to summon it is 1 minute per level.' which doesn't have an exception for mount.

Anyway, there are no build guides I know of for guardian spirits specifically, but it'd be much like a summoner's eidolon in terms of useful feats. Piranha strike is probably a must. If there are other PCs it might be fighting with then teamwork feats like outflank are good for both it and the PCs (assuming they can be persuaded), if it's only going to be fighting with other summoned monsters then forget those.

Spell-like abilities are also based on charisma so you could look for ones that have good spells for you. There's always the classic recommendation of a faerie dragon because it can use wands.

Instead of an improved familiar I would choose a fey-touched regular familiar because it is able to transform into humanoid form and wield weapons.

edit: you could even disguise it as you :)

The silvanshee Agation is a support summon and it's awesome, since his lay on hands scales up with his HD. To me a summon guardian spirit feat is the strongest feat in the game if used with a silvanshee. Let me explain with a 3rd level slot you get at least 6 use of lay on hands (1d6 too bad but it's still useful) on top of that he can give you a +5 LUCK bonus on all saves for 10 min, he can mist up and has true speech this alone is insanely useful. Then as he progresses he can became only more insane as 3rd HD feat pick Hands of valor which allow him to grant a 1/day bonus of his CHA on all attacks to an ally that has been target of his lay on hands for 1 min this is HUGE it can give your caster a better bonus to attack than a full bab class and if you want he can give it to them too. From that point on only his cat luck ability, hands of valor and lay on hands ability make worth any level of spell slot used to summon it. For the other feats you can go as you wish and it will still be good for example giving him more lay on hands feats to boot his healing effectiveness, quicken SLA dimension door. Or combat feats to boost his offensive capabilities, heck i even toyed with the idea of giving a guardian summon the smash from the air chain feat and keep him close but that's not for a silvanshee. As more combat focused option the tidepool dragon looks fantastic but it needs to be comboed with the proxy summon feat to give your summons the share spell ability, so you can turn it in a fucking medium/large/huge dragon but any beast shape will do the trick also opening you up a lot of other buffing option like mirror image, blade's tutor Spirit, displacement and so on. This makes the use of the rod of giant summon less mandatory (it's still super important but not so much) since you turn him into a fixed size whatever you want (still good for str and con bonus but not improved weapon DMG). Also I'm not sure if the rod would work with the mount altered summon trick. You probably can't turn a guardian spirit into a melee power house but you can surely with 1/2 buff spell can make him decent by himself to be a support/off tank to the party and take some heat off them while still doing a bit of damage with the smite threath, if properly beefed up it can reach the level of a good animal companion or a non min maxed melee pg in my opinion.