There's nothing special about the deadliest shot. Hero's attack = current damage + 3 levels. If the enemy is an arch-enemy, the damage is doubled, and at least one creature dies. It's just for extra damage during leveling up (although it's good for taking out high-level arch-enemy creatures). My friend advised me to also learn Rain and Deadly Cold. With Deadly Cold and a ring, you can destroy a ballista, a tent, and possibly injure someone else in one cast. If your hero is Wingael, you can use Rain to hit-and-run yourself. Or, if your opponent likes to move around with a ballista and single demons, you can take them out with one Rain spell, including their hero (even Defense won't help). Now, I'll try to give some advice point by point:
1) The main thing is to have archers built, and if possible, also druids. Usually, my build depends on the hero, but in general, the following skills are useful: racial with all abilities, Deadly Cold, Defense (mandatory!), Luck with Soldier skills, Logistics, Education... there's plenty to choose from.
2) You can level up Defense and Chaos magic. If the terrain is grass, you can take Native Lands and Boots of Speed/Tactics, so most of your units will reach the archers/mages quickly.
3) I wrote it above.
4) See point 2.
5) It seems like it's not possible