About Mercenary Discount. I have the Advancement max out and if I get the 4 techs it's 72% discount. The rare only brings that up to 74%. Considering I buy most of my mercs late I generally have at least 3 if not all 4 on larger levels. It just doesn't add much.
2 comments. Merc Discount to Epic might be good, but also raising Additional Mercs - this is actually a sort of Merc discount as well, b/c it gives you more mercs at each camp, including more of the cheap mercs for the first 10 or so merc camps. So you'll approach a point where you don't need army camps much at all to clear a level, just get what you need from mercs, caches, JS & hospitals, and eventually if you max it out, will be able to leave mercs in the expensive camps behind = less crafting time to get enough money to clear the level = faster clear time. I'm only @ 88% additional mercs now, but when I get it 150%, I suspect I'll be able to leave all the expensive mercs in the later pools, not upgrade hospitals as much = save money & time = buy more mercs = finish faster. As as Jz pointed out, finishing faster means getting less benefit from all Advancements and Artifacts that are time based like ACB, BMB, TMB, so need to focus on event driven buffs like buy mercs, caches, hospitals, speedy crafters, etc.
My last level before 2nd Ascension was Netherlands, and I had 62B drafting on it, army camps 23B, merc 373B, hospitals 417B (I pretty much maxed them all out), JS 193B. Heavy focus on mercs, and it's working out for me. Drafting earlier gets me that 60B draft and even still I thin it's too low, I did leave a lot remaining in the pool, I was still getting 2B drafts at the end of the level.
Also, if you're only buying @ end of level, you're missing out on opportunities, esp if you have Discount Mercs maxed out. You're missing the value of buying the cheap mercs early on and getting more bonuses/caches/crafters/recipes/etc = more money = faster clear. Also, more mercs bought = better drafts. If you end your level still getting massive drafts, it means you've left armies in the draft pool, and you could have moved those merc purchases earlier and got (just example #'s) 20-60B additional benefit from the draft pool.