qaccy
Emperor
I am in HMA with 114/61% (Zeus and Aachen).
Against 160/160% units I am stuck at 22/40.
In trading I finished 40/40 (with 10 diamonds).
At my level, we can actually go much further by negotiating, it may be different in SAM with 400% ?.
Fighting has a pretty clearly defined difficulty to it because at least one of your units is required to survive the battle in order to win, and diamonds cannot be used to get around this. Knowing this, there can definitely be 'impossible' battles where you simply can't defeat the enemy before they defeat you.
With negotiations, nothing is really 'impossible' because diamonds can always be used to get around both the resource costs and the limited attempts. However, most players are not looking to spend a ton of diamonds negotiating, or even running out of goods, so attrition does still limit negotiation about as well as it does fighting. It's just harder to notice when players have huge stockpiles of goods...until those goods run out because they performed too many expensive negotiations. The amount of goods you're able to produce to negotiate with will determine how much progress you can make, just like your military bonuses determine how many battles you can do.