Challenge is going well, thanks for asking
I was getting a bit frustrated, because I could see I really needed to defeat some new continent scroll guardian in order to progress, and it looked immensely difficult. I had Dersu scroll earlier, but with tactics which I am not very proud of - enemy army was mostly slower melee, so I just grabbed a few dragons and flew around burning them, fooling shooters with nature call etc. Well, it did the job and that's what matters. Bosses? they will be done at the very end of the game, although spider and frog maybe earlier. Heroes? I've done only those which I could kill comfortably regardless of their spells - Odin, Spirit of Sorrow for example.
But I'd like to share with you fight for Nameless scroll on Umkas. so here we go : first, I moved an enemy around a map a bit to have centre blocked. It's not great but at least two hexes were occupied and that's what I needed. I could not let those enemy units with speed 3-4 reach my back rank, since rune mages had no poison resistance, summons were fragile, and any unit attacked by crit from hyenas was a beyond fixing (if not killed outright). I also needed to keep Ents at the back, so they do not use their nasty distant attack. I had allowance of about 10 scrolls I think, so I grabbed few fear scrolls, demon portals (which a better than nature call - they block a hex for 2 turns), and time back scrolls (that's why I grabbed level 3 vampires, not level 4 ancient ones). Droids were obvious choice, vampires were chosen for poison res., life draining, no retaliation, but due to enemy numbers they were doing more of finishing jobs while keeping numbers high. So three centre hexes were held impregnable by 2 x rep. droids and vampires. 2 hexes at the bottom of the screen were held by summoned spiders + demon portal. I casted fear on hyenas so they just have nothing to attack behind the barrier. The only unit sent forward was sturdy droids which pulled thorns and worked on them. After initial summon rune mages just kept firing on hyeans in order to sheep them. I also needed second summon to be thorns which were kept at the back and once weak enemy stack were down, I overwhelmed the centre with summons. There you go - It is all combination of previously mentioned tactical finesses and at the end it worked.