If Barik is hugging his Turrets, use fire bomb. The turrets cannot be moved once deployed, so he will likely abandon them if fire bomb lands on or near them.
As Maeve, try jumping on top of your enemy to confuse them. Players do not tend to aim directly up, so the moment of delay is enough to land a few shots on them.
You can re-position yourself out of the AoE of Willo's Seedlings using Rocket Boots, though you may end up sacrificing some Turrets and Barricade in the process.
Nearly every Barik out there has a healing card in their loadout. It provides him a heavy sustain. Cauterize is an all around counter that also works against him very well in that aspect, even without a healer on enemy team!
Ash's ult is powerfull and easy to counter(all cc abilities) but you also can make her not even use it, like hook with makoa , torvalds ult , Jennos grip, Shalin's and Cassie's rmb arrow can block her 'Ult jump' or move her ult to other place