These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Demonic Spiders: Energy golems. They can't move and only have one attack, but it has infinite range and can hit multiple targets at once. If you engage them, make sure to take them out before they can get another turn, or you're in for a world of hurt.
Game Breaker: Dorgon. His basic attack skill has unlimited range and can hit multiple opponents at once. You can use him to easily snipe weaker encounters before they know what hit them, and soften up harder ones before your main troops can engage in melee.
Since he's an energy golem who's on your side, this should be a given.
That One Attack: Holy Blast, used by Nelis. The boss only uses it at close range, but it deals a tremendous amount of damage to the target, as well as dealing minor damage to everyone on the battlefield. Fortunately, the boss seems to prefer using a less-damaging attack as long as you don't get too close.