Theory and Practice

Um... the problem is that you need a lot of skills for this.

On many maps there is a prison with an upgraded Lord - underworld of battle, Lezkandra for example - we boost scouting - give him a rod/scroll and voila! - he has reserved his spot in the finale;)

It's not that easy to find a Rod. Since we won't be boosting his magic, a volley of Fire Arrow or Ice Arrow would be very weak on its own, so you need to find a Rod... at least of Fireballs (though still weak), and maybe a scroll with Lightning Bolt at least.

When leveling up, even without magic, the spell's power increases with each level -
Ice Bolt/Fire Bolt at level 15-20 - it will be around 130-180 + basic/advanced magic - and it's already 150-250...

And with the general huge variety of scrolls, finding exactly what you need is not so easy.

hmm... still, it's not hard to find something - mage towers, monastic havens, Dragon Utopias, just on the map...

I agree about Dispel and Cancel, but again... if you catch the scroll ;)

Dispel/Cancel works against 100% resistance anyway - from any mage - I meant removing Immortality with another mage, then the Guildmaster casts some debuff, and then we hit physically

So if you're going to boost a Guildmaster, you need to be at least 80-90% sure you'll find the right spells along the way

something will turn up - in this case anything will be useful ;)

P.S. I'm not saying you should always boost such a hero - but if we know we're playing against Barbarians, for example - with such a hero the chance of victory increases.

Added after 23 hours 24 minutes
Tested the Guildmaster. Here's what I got:

took a level 15 hero (533 Nobility+55 Scouting - theoretically need 19 levels to train from a Lord, or 20 levels from a Thief, but in reality you can get a discount of 4-5 skills from altars and universities, guilds). so...

give him a rod/scroll of Fire Bolt - it deals 135 health damage.
the increase is 5 damage per level.
with Ice Bolt/Fireball it's a bit worse - 108 damage, increase +4 per level.

now give him basic Chaos - no changes ... basic Spellcaster + advanced Chaos - no changes ... not it - you need Pyromancy, or Sorcery and get +20%
i.e. 162 from Fire Bolt. and if basic Pyromancy+basic Sorcery / advanced Pyromancy / advanced Sorcery, then it's already 40% = 189 - that's significant. Though 33% resistance still remains, but still, dealing 126 HP damage to a Grandmaster of Battle is not so easy with a physical strike.

so we get that you need 3-4 Chaos skills (variants: 1010, 1001, 1020, 1002, 1011. and if you're lucky even 1021, 1012, 1022!!! )
and we get an excellent "armor-piercing round" (c) ZYBR.
well, as for the Arm of Death artifact, there's nothing to calculate there - it will kill.