I'm B1, so ignore me if you want, but I'll help if I can.
Xin's primarily role, like Vi's, is skirmishes and zoning/diving the carries. Both function better in 3v3s rather than full 5v5s. Part of it is being able to get to the carries, which is something that Vi ult and Xin E basically solve. You dive in with Xin E and send people into your team or just split them up so your team can clean up. Dive the ADC or mid as Vi and try to zone them out or kill them with a secondary diver. Both function better with a secondary diver or tank to help them get to the carry.
For builds:
Xin likes stuff like
,
,
. Red smite is helpful for damage since it deals a lot of true damage and you auto a lot. After that it depends on what you have and what they have.
is almost certainly your MR item of choice as it helps the heal off E. After those two items plus situational boots, it's pretty much situational tank items.
is probably #1 just because you tend to live long in fights as long as you have the stats and heals, so the passive burn does a lot for your damage. Other decent items would be like
for additional AD champions on your team,
for
,
,
,
,
, other ADCs in general,
for better E initiates using the MS,
works as a good secondary item with
, moreso because you can use the passive for the atk spd slow and the Randuin's active fairly well,
is pretty much secondary to most other armor items and should be used if you don't think your damage alone is enough to win.
is a decent diving survival item and can give you a solid damage buff.
for added dive protection and MR with your
. You also want to always make sure you prioritize health along with the armor or MR, like the items above have or use
. Armor and MR function as multiplication stats off your health in a sense.
Same build path concepts function for Vi, only difference is I personally dislike
because it can be harder for Vi than Xin to stack just because she doesn't have an Atk Spd buff unless you already proc your W 3 times, which means you've already gotten 3 of the 6 Cleaver stacks.
is a better damage item overall and still gives you the 20% CDR, which is lovely.
is a little less desirable than
just because you don't auto as much as Xin, plus warrior hits like a truck.
Best thing to remember when playing
and
is you are the secondary engage moreso than the primary.
Primary engage is usual heavy AOE CC abilities or pick based abilities:
combo,
R,
R,
Q,
Q,
R,
R,
R,
R,
R,
E+R combo (RIP that),
R,
R, etc. Stuff like that is what you use initially to start fights, then you follow them up with Vi R or Xin E+R. Using the secondary engage second ensures (sort of) you don't get blown up. Most of these type of engages work well when backed up by primary engages that help them get in there, such as
and
R. These enable things like
R and
R, for example, to be that much more efficient. Some secondaries function fine on their own,
R for example, but they can be that much more potent with primary engages starting them off.
Hope this helps mate.