In my experience guards will kill you unless you are a nightblade and manage to get into perma-cloak. Their damage escalates something like 10 to 20 seconds into a fight with them. If you don't flee and manage to escape, they'll one shot you. I thus don't see the point of a guard damage reduction set, but I haven't worn it, so don't know what it does exactly. Maybe some combination of high mobility (Streak?) and invisibility potions can work, but guards have a habit of uncloaking you. Even as a nightblade you need to spam cloak several times to shed aggro permanently (and get to an outlaw refuge). The basic reason that happens is when projectiles are already in flight and you cloak. You still get hit and get uncloaked again. Some NPC DOTs also constantly uncloak you until they wear off.

Outside of being a nightblade, you could be a stage 4 vamp, which makes you invisible after 3 seconds of sprinting. Invisibility is the same as NB cloak (Shadowy Disguise). It sheds all aggro as NPCs can't see you anymore. This is not the same as crouching. Crouching is weaker, but accessible by any class / build.

There are some sets / skills that help with movement in crouch. Being a vamp (stage 1) helps with this again. The alternative is a crafted set, Night's Silence. Some sets also reduce your detection radius and make you harder to see in Crouch, but only invisibility (cloaking) reduces your detection radius to effectively zero.

A set that may be useful is Darloc Brae, as it reduces your detection radius and gives you a lot of mag and stam sustain while crouching. This allows you to run in crouch forever via the stamina sustain component and it helps with cloaking (Shadowy Disguise) or speed in Crouch (via casting Race Against Time (Psijic)) via the magicka sustain component.

Finally you could simply try queuing for Cyrodiil or Imperial City to extricate yourself from any fight. You'd need to work out some quick access key for that, maybe an addon. It's a PvPers trick when they want to save their Tel Var or just stick it to the opposition zerg. I don't know whether there are justice system guards in the PvP areas. I'd say you're probably in the clear by doing that.

bahrahas curse i think has a line item that does give you reduced dmg taken from guards by 20%, its shared with a line of stamina i think

but like fred suggests, its usually better to not aggro the guards at all if you can help it, my stealth toon runs 3 pc night terror, 5pc darloc brae, and ring of the wild hunt and is a vampire so im at speed cap while in stealth and can spam cloak

i also dont know what is classified as a guard though, the invulnerable ones in towns, or do the npcs guarding keeps/resources in cyrodiil also count as guards too? lol