New House Rule: How to Eliminate Magic Missile Exploitation of Minions
Under the current official rules of fourth-edition Dungeons & Dragons a first-level wizard will automatically defeat a minion of a level much higher than him or herself, even a 20th-level minion, by using a simple magic missile spell. This is because magic missiles, which typically cause three to six points of damage, automatically hit, and all minions take only 1 hit point of damage, even 30th-level ones. Thus, a 30th-level minion can provide a party of 1st-level player characters with 4,750 experience points as soon as a wizard drops it, which means the party could theoretically leap to 2nd, 3rd, or even 4th-level by simply destroying a couple such
1) You can simply bring back the original rules for Magic Missiles as outlined in the first release of the 4e Player's Handbook. Under this rule, wizards must make an Intelligence vs. Reflex attack to hit targets, whether they are minions or not. This certainly takes care of the exploit issue, but it does not provide the same "feel" of what a magic missile used to do in previous editions of D&D. This "feel" is why Wizards of the Coast changed the fourth-edition rules in the first place, so that magic missiles now automatically hit like they did in previous editions of the game.
2) You can implement the following house rules. This system, I think, is a good compromise between the two methods on how magic missiles work. It avoids having minions fall too easily to a magic missile exploit, but it also provides the same feel of a magic missile from previous editions of the game.
If the target is a minion of the same level or lower than the caster, or it is a creature with a compliment of hit points, use the current official 4th-edition rules. But if the minion is of a higher level than the caster, the caster must make an attack roll to hit as outlined in the original fourth-edition rules (Intelligence vs. Reflex). In this case, though, no negative modifiers apply to the roll (such as concealment or cover), and all positive modifiers do apply (such as combat advantage). This gives the caster a better chance of hitting with the missile than he would with other powers, and a better chance than other player characters would have with other weapons, but it is definitely not guaranteed.
The reason behind all these rules, of course, is to make it less likely for a level 1 wizard to instantly kill a high-level minion with one magic missile.
These house rules have been tested many times by our own group, and they work very well; it does not unbalance the game or slow it down or add unnecessary complications. In fact, if anything, it adds more balance. Of course, in the above example with the 30th-level minion, I was exaggerating to illustrate my point. Most DMs would not have PCs go up against minions 20 or even 10 levels higher than the party. On the other hand, I have frequently used minions in my own games of four to six levels higher than the party's level as a way to make battles more interesting and to speed up play; this discrepancy in levels, at times, has been as much as eight levels. In theory, such creatures shouldn't drop instantly when a greenhorn 1st-level wizard flings a magic missile at them. To fix the problem I think the above system works great.