I assumed everybody uses diamonds in the complex negotiations. Is that not the case? That's how I blew through my diamonds, and it's why I'm not happy with the trash reward of 25 diamonds as opposed to multiple blueprints or rare buildings.
I only use diamonds for negotiations after
(1) I've activated the tavern boost for a 4th turn (I'd focus on fully upgrading your tavern before using the silver for anything that's not an event requirement)
(2) I know the answer is within 20 diamonds. Anything more and I'd rather quit the negotiation and start again. If I'm running low on goods then I'm not going to get much further in the GE anyway and it'd be worth leaving it there for the week unless I'm specifically on the encounter that's got a reasonable diamond reward (and can afford the diamond loss if I don't receive diamonds).
(3) I can't get any further fighting
(4) I would be more hesitant to use diamonds prior to lvl 3 unless I'm really confident I can beat at the very least the end of lvl 3 no problems, preferably lvl 4. Simply because once I'm at lvl 3 I know looking at the clock whether or not I'll be completing the GE far enough to recoup those diamonds. If it's any earlier then that then I risk not getting my investment back unless I can continue without spending any more until I reach lvl 4. Which if I'm spending too early would indicate I'd struggle to do so as I'd only be negotiating if troops failed.
Amount of Goods needed for negotiations: 200 of each current good if you want to break even for one weeks GE negotiating lvls 2-4. That's 10 goods buildings minimum. You'll also need stock of the previous age.
Fighting GBs
In EMA I have a lvl 6 Zeus and a lvl 10 CDM. That gets me to the beginning of Lvl 4 by fighting with only 2 or so negotiations if I can't quite get through certain combinations. I do use rogues with Heavies, but prior to that I was using normal troops and was losing just as much as when I switched over to rogues. The main difference is now I use a Alcatraz to generate my troops as much as any troop building.
Now the CDM is a Late Middle Age GB, two ages above yours. SO I would focus on getting a Zeus and Cathedral of Aachen up to lvl 10 unless you want to get advanced GBs (in which case use FPs to buy the goods from another player for building the GB, and make sure it's someone you trust will be honest. Someone inside your guild will be safer then outside your guild)
But then again if I wait until I get to a later age, they'll have better units then as well, right? Is the only option simply Zeus, CdM, etc and/or negotiations?
Yes. The further ahead in age you are, the better troops GE will present. If you wish to advance a age wait until GE starts, then unlock the tech in the new age. You can dump the FPs ready to unlock prior, just as long as you don't click the unlock button. The difficulty of GE is set when it starts. So this will give you one week to go through the new tech far enough to unlock current age troops while still fighting previous age troops. Of course all the rewards will still be previous age until the next week, but the difficulty will be what you're expecting while you prepare for something harder
My Industrial Age city has a lvl 9 Zeus, lvl 8 CDM and a lvl 2 CoA. I'm able to complete part way through lvl 2 just fighting with ordinary troops and only using rogues if I'm particularly short on something (no alcatraz, and 2 rogue hideouts, used to be 1 until a week or two ago). This is with 11% attack boost from event buildings as well. It'll be interesting to see how that changes once I get CoA up to match the other GBs, but even 3% attack boost can be enough to get through an additional encounter on auto battle.
