Maybe you'll first describe the way you play to beat AI so easily. In my experience, city militia on insane pretty much one shot 6-8 lvl hero.
I use hero's.. since I keep picking up collars.. I eventually take a good animal like the mauler guy (can't remember the name but not the bear!)
Umm 1 reason why I'm about to defeat the nearest AI is on turn 25.. he took his whole army and left his city
If his city was guarded and he hadn't left.. yah, I couldn't have taken him with just 1 hero here.. but with 3 hero's some ranged attacks a mauler unit.. could have definitely taken it if he was "home" (I think)
I've decided in making it harder in my present game.. I'm only going to have 3 hero's.. (I don't bother to play with troops at all.. usually)
I think if I played with on purpose just the 1 hero.. like I said above.. that probably be "pretty hard" I'm not sure if I've ever tried this? (maybe)
Maybe you'll first describe the way you play to beat AI so easily. In my experience, city militia on insane pretty much one shot 6-8 lvl hero.
Okay, let me try again.. typical game.. I try to (when doing it right) get lots of fame.. for more and more hero's.. I build only hero stacks with horses and as much "gear" as I can afford (that sh@# is expensive
) when I'm also doing it super right or going mage.. I'll get the ahh magic version of the tower spell and grab up the magic before the AI does (where possible)
I don't think there's much more to that? When I see the AI is strong.. I use strategy and (that means avoiding interaction with the enemy alot) and range them to death unless if I'm a super powerful mage.. I'll probably just magic them to death and move on... and in the earlier part of the game it's harder and I will have engage tactics a lot because if they get to my hero's.. they may kill them in 1 swoop. (tougher enemies.. can do this)
So it's a really a rinse and repeat at that point.. use tactics.. when super powerful (if mage or not) use your tough hero's to take out the ranged or the powerful one.. (and in the beginning toughness I might die the 4 or 6 tries but when it gets to no tries.. you know you've won.. I like blink a lot always try to equip that ring for at risk hero's and all that)
I really, really hoped 'battlegrounds' was some sort of custom designed scenario for a tougher game (based on the description) but so far not seeing that.. and still don't know exactly what it is..? other then some maps..
here I'll do more then describe.. I will give you save files.. in theory that'll work 
this is my last save file for my last game I quit cause I thought it was too easy..
And I always create a "commander" hero just for the ability of roads.. so I can move quickly and with speed (the roads do that!)