This is one of many strats for the engineer in tf2. Feel free to remix this to your liking!

The concept for this is to be able to defend your intelligence/point, while still being able to leave and get kills

Loadout

Wrench: The Eureka Effect
Secondary Weapon: The Wrangler
Primary Weapon: The Widowmaker
You can use any variant of these, but you still need these weapons to some extent.

A diagram of this

Key

Red = The Intelligence

Blue = Teleporter
Green = Sentry
Orange = Dispenser

Explanation

I used the tf2 intel room for this, but you can honestly use this anywhere where you have a corner
First, make sure your teleporter, sentry, and dispenser are max level. DO NOT CONSTRUCT A TELEPORTER ENTRANCE
Once you have made the layout, you can just go and attack the enemy team.
When you get the heads-up that your sentry is attacking something, use The Eureka Effect, and teleport to your teleporter exit.
Use your wrangler once you arrive, and finish off whoever is attacking your sentry.
Repeat.

BONUS TIPS

Your dispenser and sentry are a wall! Use that to your advantage!
If needed, leave your sentry-dispenser barrier, and scare the enemy out with your Widowmaker!

Images of this setup