AoS Guards on attack

Knights are elite units, the same as guards. And knights are faster and each one got 2 wounds, so with 120 pts you're fielding models for a total of 10 wounds.
Guards are strong, but to field them in large number, you have to pay. 20 Guards mean 400 pts.
You can certainly do it, and they can certainly give you many satisfactions, but keep in mind that basically every army can field something that excels in killing costly elite units, and our guards suffer mortal wounds and high rend attacks.
Necropolis knights, Spirit hosts, Minotaurs, Stonehorns, Thundertusks, Hellcannons / Dwarfs cannons / Empire cannons, Jezzails... the list is long.

Obviously, what's good for the goose...
Hammerers are an excellent elite troop (especialy if backed by a runelord), but 180 pts of salamanders can easily wreck 400 pts of Hammerers.

 

first thing to know: warriors are better than guards if you plan to throw big units in melee, accepting heavy losses but dealing good damage in return, while other heroes add their weight in the combat (sunblood, Carnosar and similar)
In defensive combat and employing smaller units, guards are better, hands down.
However, as said you must be aware that certain enemy units are a bane for elite troops as guards, so you must "protect" your points investment. This can be done in many ways:

avoid unwanted charges: you are slow, so the enemy can charge you easily. Surround your 2 "ranks" of 10 guards with a thin screen of 10 disposable, cheap skinks. If something wants to charge, it must clean your skinks before, or it will impact on your screen, without dealing damage to guards, that will be dragged in combat with "pile in".

Protect from shooting: increase your saves with magic or, even better, use Starlight to impose a -1 to hit to enemy units targetting the guards. This will work also in CC, to reduce the chances to be the victim of nasty effects (for example: bloodletters of Khorne inflicts a mortal woundsfor each 6+ to hit; if you reduce their to hit, you will be protected from one of your worst enemies: mortal wounds)

Scare your enemy: make your guards a truly fearsome threat in close combat, something the opponent will forever try to escape. This works really well with mixed armies (a combo of guards / mystic shield / eternity warden / Sisters of the thorn is devastating. Read the description of the spell of the sisters and smile), but can be done also with seraphon only units (keep your 15 guards in the range of 4 salamanders)


This is not perfect, there will always be something that will laugh at your tries and will just blast away your precious guards (dwarfs flame cannon, just to name one), but you will learn to know those threats, and will deal previously with them (shadowstrike starhost and / or chama skinks)

 

The only downside is that if you include Sisters of the Thorn, you must field an "Allegiance order", and guards won't count as battleline.

 

Ah, i missed that one.
Yeah, that's even better, the Sisters lose the benefits of the Seraphons' Allegiance, which are... nothing.

So, to sum it up: you can pick seraphon Allegiance, have guards as battleline, have Sisters of the Thorn in your list, and have the abilities of the Order Allegiance. All the goodies, and no downsides.


...a pity it's the same for our opponents.

 

It is allowed, and if you are not playing matched play, you can even stack mystic shields!

However, in matched plays, a 1 is always a failure, so there is no difference between an eternal starhost with save 2+ and an ES with mystic shield and save 1+... if not for the fact that sometimes a further bonus is vital (rend lowers your save rolls, or when you have things as Sisters of the Thorn, where each bonus to saves means that the effect of the Shields of Thorn applies with a lower roll)

 

in theory, you could stack everything, 'cause AoS FAQ are still valid. In practice, you will be blocked by the Rule of One: you can cast each turn a particular spell, so no matter how many wizards you have, it's just one Mystic Shield.
But you can still have nice bonuses... guards with Mystic Shield, Eternal Starhost, a nearby hero and in cover, can have a 4+ on saves.