“X-Saber” is one of the few archetypes we have in Duel Links that is actually capable of Synchro Summoning: it doesn't have to go out of its way to Special Summon monsters from the Extra Deck, but, at the same time, it doesn't completely rely on them.
Since a lot of the new “Goyo” Synchro monsters that have just been introduced into the game benefit from having Warrior-Type EARTH monsters on their side of the field and they also have pretty generic requirements when it comes to materials, it's only normal to pair them with X-Sabers.
If your opponent controls a monster(s) and you control none, you can Special Summon this Level 1 Tuner from your hand. Unknown Synchron is a great card that gives you easy access to Level 5 Synchros such as X-Saber Wayne and Goyo Chaser, but it can also be used to Special Summon higher Level monsters from your Extra Deck (if you use Powerful Rebirth to Special Summon XX-Saber Boggart from the Graveyard, its Level is going to increase by 1, giving you access to the Level 6 XX-Saber Hyunlei and Goyo Predator).
XX-Saber Fulhelmknight is a Level 3 Tuner, it doesn’t Special Summon itself like Unknown Synchron, but, once while it’s face-up on the field, it allows you to negate an attack declared by an opponent’s monster.
You can use Fulhelmknight and XX-Boggart Knight to Synchro Summon X-Saber Souza or, if one of them was Special Summoned by Powerful Rebirth, even Stardust Dragon and Goyo King.
You can use Fulhelmknight and XX-Saber Darksoul to Synchro Summon XX-Saber Hyunlei, Goyo Predator, Goyo Guardian or X-Saber Souza.
You can use Fulhelmknight and Paleozoic Canadia to Special Summon Goyo Chaser, X-Saber Wayne or XX-Saber Hyunlei, Goyo Predator and Goyo Guardian.
When Normal Summoned, XX-Saber Boggart Knight allows you to Special Summon a Level 4 or lower “X - Saber” from your hand—this is your main way of swarming the field with this deck. Boggart Knight makes it easy to Synchro Summon and it also synergizes well with Saber Slash.
If XX-Saber Darksoul was sent from the field to the Graveyard, during the End Phase, you get to add a “X - Saber” monster from your Deck to your hand; this effect will activate even if you used Darksoul as Synchro Material, but not if the monster is banished.
Saber Slash allows you to destroy cards on either side of the field up to the number of “X-Saber” monsters you control, its effect does not target and it can enable a lot of OTKs.
You can use X-Saber Slash to destroy all monsters your opponent controls and then Special Summon Hyunlei to clear the backrow as well.
XX-Saber Hyunlei doesn't have the highest ATK, so it will struggle dealing with cards such as Blue-Eyes White Dragon, but its effect can essentially clear the entire opponent's backrow at once.
There are a couple ways you can Synchro Summon this Level 9 monster: you can use three of your monsters, including Paleozoic Canadia, or you can take control of an opponent's Blue-Eyes White Dragon or Dragon Spirit of White and then use it, together with Unknown Synchron, to Special Summon this 3100+ ATK monster.
Goyo King is a Level 8 Synchro that, when declaring an attack, gains 400 ATK for each face-up Warrior-Type EARTH Synchro monster you control, including itself. When Goyo King destroys an opponent's monster by battle and sends it to the Graveyard, you can decide to Special Summon that monster or take control of another monster your opponent controls.
With 3200+ ATK on attack declaration, unless DNA Surgery or something else altered its Type or Attribute, Goyo King is capable of getting over Blue-Eyes White Dragon, Masked HERO Anki and even Cosmo Brain and then Special Summon them on your side of the field, ready attack.
A Level 6 Synchro with a similar effect to Goyo King, Goyo Guardian was locked behind a Skill with a pretty substantial drawback or a 40% chance. Goyo Guardian is undeniably a great card, it does lack enough ATK to get over some of the most common high ATK monsters you can find in PvP, but you can use Paleozoic Canadia to make up for it and just switch those monsters in the position in which they are more vulnerable.
Goyo Guardian, unlike Goyo King, Special Summons the monsters it destroyed by battle in Defense Position, making it impossible to use them right away for an attack without some outside help, and, when you take into consideration the limited Monster Card Zones we have in Duel Links, it really makes you wonder why the developers tried so hard to deter players from using this card.
Goyo Chaser has the same effect as Goyo Predator, but the Attack of the monsters it Special Summons is halved.
Goyo Defender barely deserves being mentioned, its first effect is completely useless and its second effect is difficult to activate.
There isn't much to say about this Skill: you can end up with Goyo Guardian and Goyo King in your Extra Deck or with Goyo Chaser and Goyo Defender; each Synchro monster has a 40% chance of appearing in your Extra Deck, assuming the individual chance is the same for each one of them, which is likely, and there cannot be two copies of the same card.
It could be argued that a standard X-Saber Synchro Deck is superior to this one, being able to utilize other Skills that can aid you when it comes to Synchro Summoning. Having more variety is great, but, given the limited space in the Monster Card Zones and how relevant Trap Cards such as Mirror Wall, Treacherous Trap Hole and Wall of Disruption, you are probably going to want to go into your standard XX-Saber Hyunlei or Stardust Dragon rather than a “Goyo” monster.
As I said, you have only a chance to get a Synchro Monster that could actually justify running this Skill.