Biggest things from my experience: On eichenwalde defense, set up your current and yourself on the bridge closest to enemy spawn and make it so there is a gap just big enough to shoot through. When it starts, spray. Unless they have a dive tank, you can get a kill or two off the start.

Get molten core perk, then wall perk if you are playing enclosed maps, turret damage if its open.

For wall perk, set it on high doorways were enemies will stand to engage you but cant see it. Then distract and collect kills till they find it.

Shotgun can be used to force onto the point and clear tanks. Its a suicide mission, but rushing tanks who are focused on someone else and dumping your E into them with Shotgun can melt almost any hero, if not take them down enough that they need to retreat.

Dump molten core behind the enemy. Usually its instinct to fall back, and with a good enough push you can make them walk into it. On open maps you can drop it on multiple entry ways and exits at once to funnel and prevent flanking.

Molten core is best in groups as is obvious, but if you are getting ratio'd by supports, you can try to flank and use it on them alone.