Simple, just upgrade Health and Shield "equally" when you level up and make sure you always get the Regen perks when you can too. You only need Flame Turret and Plasma Shield for your offensive/defensive abilities don't worry with the other offensive/defensive perks. e.g. Gatling Turret. As they are not as good. However the ac110 is also pretty good but I think the Flame Turret is more helpful as it kills and distracts the enemies while you can recover.

Also make sure you always get the perks that are the most useful e.g. books1,2 and 3 to get more XP, or radioactive to hurt/kill enemies that get close (Useful while you have invincible on as you can just run in that horde and mess them up!)

I may also recommend eventually upgrading Accuracy a little too! Mine is on 90 atm but may make it 100 in the future but not any more than that.
Here is my stats atm for mine.

Now at first it will be difficult as the Engineer is very weak when you first use him. But it will get better when you upgrade his health/shield. Don't be tempted to upgrade his regen as your wasting your points on 0.01 which just isn't worth it. Apart from that I can't think of anything much esle to go on.. save your plasma shield for an emergency and recommend picking up and saving Ion Beam for the bigger enemies when they appear if you get the chance!

Hope you liked the guide and happy hunting!

P.s If you wan't to unlock a couple of new characters select create new and in the name either type..
UNLOCK_MARKIPLIER
UNLOCK_JACKSEPTICEYE
ENJOY!