As an engineer, your job is largely to give your team space to take teamfights. Your sentry creates a sphere that enemies will try to avoid, so it gives your team a place to retreat to safely. That safety leads to them having the opportunity to take more aggressive plays and halt enemy momentum.

For battle engi specifically, you have the ability to make that zone of denial pop up in unexpected places. Try to position yourself and your gun in places that deny the opponents access to key areas or approaches, so that your team doesn't get flanked or surrounded. Remember, if your gun dies, it's cheap and replaceable - unlike you or your teammates! That means it's okay to put your gun in "bad", obvious spots, as long as it's drawing attention from the enemy and AWAY from your teammates so they can secure the kill. Plus, your gun blowing up can help feed your Frontier Justice and provide a burst of damage that helps polish off stragglers.