DM usually gets surprise rounds. Players then are less inclined to 1 shot things ... because then said same combo is fair game on them. Gentleman's agreements are a necessity for fun, not a luxury.
That's exactly backwards. If the DM is typically having the enemies get surprise rounds, that means you have even less time to kill them before they kill you, so you need to be able to one round things even more.
No... because if you do then those tactics are legal. Therefore the DM might use them on you.
So that's out. Now suppose you chose to be smart and not play in 1-shotting campaigns. The monsters getting the drop on you? You can survive it. You attrition them right back but start to play smart meaning on subsequent rounds you aren't as vulnerable as before while the enemies still are. Eventually you win but not after reducing your resources.
If your DM plays monsters as smart or smarter in tactics, that's a massive EL boost, making on-even CR'd creatures usually not winnable. Insert Kobold jokes here.
Facepalm. I hope you are joking, and did not honestly just claim that playing enemies intelligently constitutes some massive buff given that most of them are indeed intelligent.
Level appropriate enemies 1-2 round you. If enemies get a surprise round, that means round 1 is round 0. You have one round left. Make it count. And don't feed any BS about it being the DM's fault.
People complain about spells, but simple attacks do exactly the same thing. At least when it's enemy on PC. So even if the enemies are stupid casters or not casters at all, they're still going to be taking people out quickly.
The only problem is that PC non casters don't have rockets. They can't keep up without specific builds and optimizing. Claiming spells are the problem is exactly backwards, wrong, and that way lies 4th edition which we already have.