No one knows how to play with an Abathur on their team. The correct thing to do: give up the first few early objectives but delay them for as long as possible with pokes and safe play so that Abathur can snowball an xp lead to Level 10. What you’re team will do: fight 4v5 over and over and over, lose every objective, feed the enemy team kills, and completely negate your split soak. Fortunately, once you fall behind, Abathur offers basically no comeback mechanics.
This strategy is particularly weak on small maps like Tomb of the Spider Queen and Dragon Shire. Your team will also suffer greviously if they have poor waveclear compared to the other team, as the lack of a 5th body will mean that at least one lane is shoved in at all times. Pick Monstrosity as your Heroic if you want an additional challenge.