To earn a B grade on this challenge you must reach a score of at least 6,000 points
Details: Bonus damage when survivor overlaps with turret is increased to x3.
This is definitely the easiest of the challenges as not only are there no detriments to your game but you also get a 3x damage multiplier instead of 1.5x. You also only need 6,000 points so chances are you won’t need a guide. However, here’s the method that I used just in case you need help.
First I started with a Rifle turret next to the top left corner of the mud patch, simply to help with any stray Winged Horrors that get through. Next I build three Quakemakers around the spawn. I then started upgrading these three turrets to level 4, starting with the middle, then the left and then the right. While waiting for money to accrue you should be boosting the power of the middle Quakemaker.
After this I build a Flamethrower between the middle Quakemaker and the Rifle turret. I began to sit on this instead of the Quakemaker and levelled it all the way up to 6. Whilst continuing to power this up I upgraded the Rifle to level 4. Finally, I build another Flamethrower to the left of the first and upgraded that to level 4.
My last step was to build another Flamethrower to the right up to level 4, upgrade the left Quakemaker to level 5 and finally to upgrade the left Flamethrower to level 6. At this point you will have your B grade. I left my game and it only ended at 8,000 on an A grade.
https://imgur.com/kAtlIXV