The Gift Bag is an OFFICIAL "mod" (if we can call it that way, as it only adds new items and new sets), made by the developer of KBTL/KBAP - 1c/Katauri as a present for all their supporters and players. As such, I consider it legal, as it is not fan made and it does not shift the balance of the game in favor of any hero class. Here's the thread for you:
http://forum.1cpublishing.eu/showthread.php?t=12174
It will be hard to find a scoreline in which the damage done by spells will outweigh the damage done by troops. Troops exchange strikes all the time (and there are 5 of them, unless you play with less), so by the end of the game, they will deal at least 20k damage per turn (actually, 20k is really, really low, but it is a good number to start from). With spells, you can barely match that, unless the enemy stacks are well grouped for a decent death star cast, or physically weak for a 20+40 mana geyser cast. In my own experience, the best I got from a mage was 34% damage done by spellbook, 2% dragon, the rest troops.
An option for the spellbook to outweigh the troop damage can be what you saw in the video - play with one stack, armageddon all the way and that's it.
EDIT> As you said, in boss battles, you must use direct damage. Well, Baal has 253332 Health on impossible. K'Tahu has 172000. Zilgadis has 154000. Dersu frog has 80000. The Spider has 60000. The Driller has 14000+28000+56000=98000.
That's a total of: 817332 damage you will need to do with your troops. In order to have your spellbook damage on par, you will need to inflict that much damage with it in advance. That's quite some damage to catch up, don't you think?
It would appear that you want to have a playthrough where you do the most damage with spells right from the start. If I am correct in assuming that, there are ways to do it here, and you are by all means free to pursue them. Regretfully, Time Back is a spell now, which means you'll be wasting quite a lot of mana, but it is doable if you use the mana accelerator pet dragon ability to regain at least part of what you waste and calm rage for some more.