"In honor of our fathers" - "One Khan, One Clan" [Campaign 3, Mission 2]

I did pretty much the same thing, except I upgraded Assassins to Executioners. I used them specifically for melee combat; they are very powerful in that role. As for upgrading Shamans, I definitely agree with upgrading to Daughters of Heaven: essentially, the hero gets the Chain Lightning spell for free, and with a free movement point to boot. But I still think Centaurs are best upgraded to their second tier.

Well, in melee combat, if you're not just defending your castle, then Executioners are definitely the way to go. Otherwise, they're useless. But why are you putting Centaurs at their second tier? Their damage is weaker, even though they retaliate with full force. What's the point of letting anyone get close enough to attack the Centaurs and deal significant damage? I try to come out of every battle with minimal losses among my archers. Let others die, but not the Centaurs, because they are incredibly powerful, especially with maximum Blood Fury, when there are a couple of hundred of them, they mow down everyone, and if you add Direct Hit, it's game over. Let me tell you, with well-developed Centaurs, no other archer unit can compare, not Dark Elf Witches, not Human Archers, not even Elves with their Double Shot. In short, Centaurs outperform everyone, and ideally, you should combine them with the Ring of Life (health +2) and take the Defense perk, which will give them 13 health instead of 9, and with Battle Fury, they are simply overpowered