Like everyone said this is an absolutely solid team.I lost terribly against this team used by some peoples on the ladder a few times.But then when facing and beating this team with two of my HO teams I noticed a big threat to this otherwise almost perfect team.Speed boosters can sometimes sweep through your entire team.Particularly Sheer Force Landorus with Rock Polish.Double Dance terrakion can be problem as well but they are rare and you can play around them if you can anticipate their sets.
Landorus can set up on (1)Dugtrio, (2)Ninetails when paired with TTar/Hippo/politoed,(3)Genesect locked to Tbolt,Forcing out(4)Victini locked into Fusion Bolt/Brick Break.(the numbers represent from most common scenario to least common scenario).It can then OHKO/2HKO every member of your team with a combination of Earth Power+HP Ice+Psychic.(Only Epower+HP Ice if Rocks are up).
Terrkion can set up by forcing switches and in a similar scenario to (2) of Landorus.With Rock Polish and Life Orb it can outspeed and OHKO everything on your team bar Venusaur ..
So in my opinion SF RP Landorus is the biggest threat to this team ,although it can be sometimes stopped without rocks up but then some SR layers such as ttar,Mamoswine etc can eliminate Xatu and TTar can tank a hit from Duggy and proceed to lay rocks.I don't know about Mamo though since i used Icicle Crash on my Sash mamoswine for more power which is seldom used.
For this reason i am thinking of suggesting Sp Def Forretress instead of Xatu with Gyro Ball,HP Ice,Volt switch and Rapid Spin but that way I might open up more holes and Hamper your Offensive core.yeah sorry for only pointing out threats and being unable to provide a suitable suggestion.Anyway Luvdisced.