Helldivers 2

What keeps happening to me that I absolutely cannot stand is dying from friendly grenades and airstrikes. And Arc Throwers. So many lvl 35+ people murdering me with Ark Throwers aimed at Chargers when I am trying to flamethrow them...

I don't mean to sound like I'm blaming you for getting TK'd, but it sounds like maybe you may need to defensively reposition yourself in firefights. Not necessarily in relation to your enemies, but in relation to your teammates. If you see they're using a lot of AOE weapons, you'll probably want to make sure you're not standing downrange of them. This also means making sure you don't allow enemies to get too close, as aggro'd enemies will always attract fire in their direction - usually from well-intentioned Helldivers trying to "save" you from a Hunter... with an EAT.

I find it's a good idea, in general, not to cluster up too closely to your teammates either (as this makes it easier for a swarm to wipe you all at once), so make sure you're giving your teammates a moderately wide berth at all times. I don't have any exact measurement, but in general I want it so that if I'm looking straight into the horizon, that my teammates are far enough away that their feet are in the top third of my screen or higher; I know where they are and what they're doing and can provide cover fire if they're getting into a hairy situation, and I've got a safe distance where a stray grenade isn't as likely to insta-gib me.

I try to always maintain a pretty wide "bubble" around me during engagements, where I don't allow enemies or friendlies inside. If I can't keep my bubble clear, I backpedal until it is. It's definitely slower and there's not as much bravado to playing this way, but it keeps you alive.

It's all kinda like riding a motorcycle; you can be doing everything correctly, but the biggest danger to you will always be the other people on the road with you. Never trust that the other drivers know how to use their mirrors, and never trust that your fellow Helldivers know what "splash damage" means.

Also, this may seem counter-intuitive, but if you're using quickplay, I suggest going up in difficulty. Generally, the players on the higher difficulties are a bit more experienced and have a better game-sense. It's pretty rare that I get TK'd at 6 or above, but if I hop into a level 3 quickplay lobby because I'm bored and feel like helping some newbies farm up some samples, I find that I'm way more likely to die to friendly fire than anything else.