Coop required or do I suck?

[Majestic Twelve]

Tried doing the main story mission on that third area with normal difficulty. I got trounced hard even using the area artillery on RD. And, naturally, right before evac came I had to leave the landing pad as I was in a fighting retreat but the numbers were overwhelming so I had to jump down. Prior to this point I had done the prior missions no sweat even that first difficulty 3 with the boss spawn. This was a whole 'nother league.

So, does the game require coop at certain points or is this the game telling me to grind?

[HeathenSW]

Game wants you to grind. I completed everything alone, though it took quite some time - what you describe is quite the difficulty spike, basically. What helped me was focusing more core power - around 1.5k should buff your turrets enough, I think.

[Majestic Twelve]

1.5k core power is a hell of a lot of grinding. I had 133 and that was enough till then. :)

[Annihanni]

My endgame Base has almost 6k core power although as far as I know damage only goes up to +300% turret damage at 3k core power and the x-boost field also does not grow beyond 3k core power but it is said that it stacks up to 10k and I don't have anything else to spent my resources on so...
Anyway, stack core power. Use generators for +10 core power each or if you have advanced batteries for also I think +10 core power.
Another suggestion I would make is use 6 smart mortars, beside ciws they are really good and efficient as the shells are like homing missiles and therefore not wasting amunition. Set up 2 mortars to the front as well as to the left/ right. I still have those 6 smart mortars on my endgame base.
And then you should also set up your turret range to max so that they start fighting enemies earlier and don't get as close to your base.
If you stil struggle you either have to little turrets or too many hard perks that make enemies strong.

[bessergehtnicht]

You will need good Equipment.
First the Gun. Elemental Scatter Shot is the first choice. This will melt down the small trash.

Then your Armor. Engineer Helmet will load your Turrets with up to 75% free ammo.
Mechanical Gloves will let you go to far awy places or boost your way out.
levitation boots will let you fly. (try out the CIWS guns ;) )
Weakspot damage will help against Kronos, together with electro ammo.

Your Base will need 3 things.
1. twin barrel 30mm and CIWS. twins for the heavy and large and CIWS for missle and medium.
2. stack up core power to 2k+ with batterypack, material storage, or small generators.
(for generators use pillars. U can mount 8 of them to 3 Pillars.)
3. autoloaders and/or a lot of ammo makers.

Use what you get !
1. Use the market. Put in People in base management up to level 6.
There are specialists for every part of your base.
Then you can get most of the stuff at the market, to build up your base fast.

2. techpoints.
Pretty easy to get at the infinity siege mode, or the recovery day.
infinity siege is easy up to wave 10.
Start, command mode and bullet time, find the batteries on your way, harvest some tank wrecks, put down the first turrets, use your elemtal scatter shot to finish the first wave,
produce ammo in command mode, and so on.
You can choose what you will face in the next wave ! Don´t make your life harder with +30% more life/damage or sandstorm !!!
Use 1-2 material converter to make resources from stuff you daon´t need, or you can find elsewhere.

Take only those small xenium cubes (they will give you 1 point each to boost your research and are easy to take out with your drone.) , and exotic drops.
drone cores, nanochips, equipment, such things.

3. Snow ruins are your way to heaven !!
Take the first spot, quick collect the rest, head for those special points, stack up resources and ammo there, go to recovery day.

sprint down, harvest the tanks and pipes at the defen point, put down a line of 6 machine guns and a second line of 4 twins, put down spikes and wait for the kronos.

i use a rapid core gun, with straight grip and light bullet+strong/condensed+1x torment+1x light strike.
you can melt down kronos in 30 seconds, aiming for the weak spots.
With a shield sight and in your xboost field you are pretty safe.

repeat this a few times, buy everything you need at the market and you are good to go.

Edit:
I put in a screenshot to my account, to show the perfect defend line on snow ruins.

And you can free place your core tower, something i found out much too late. and turn it 180 degree, so you access it from the backside. You only need a door frame at the second floor, cover the rest ;)
like at the other screenshots.