Mustrum_Ridcully
Legend
If you hit them.Minions actually don't work the way described above because that kobold minion would still die in one hit from that 1st-level party that just came in through the other door in the room.
A monsters average life expectation is based on its hit points and the likelihood of hitting it. In a way, you could classify it with a formula like "Hit Points x Defense", with a few extra weighing terms. A Minion at higher level shifts the factor from HP to Defense. In a mathematical perfect XP system, one could actually try, this mythical product value could be a Minions XP value.
By RAW, a Kobold Minion is always a Kobold Minion, and a Kobold Skirmisher is always a Kobold Skirmisher.Monsters in 4e exist in some Heisenbergian state wherein their "normal" or "minion" status solely depends upon the level of the first party to step into the door. Once that state is fixed by party A, when party B enters the room, the "role-playing handwave" of why minions make sense is destroyed: their existence is solely a reflection of PC power.
The selection whether you pick a (hypothetical) level 9 Kobold Minion instead of a Level 2 Kobold Skirmisher against your 10th level happens out of the game during a "metagame" phase - your encounter or adventure design.
This is also a formidable way of making them more difficult to run. If you give them hit points, you have to track them. One of the things that makes running large battles in D&D 4 easy is that you don't track the Minions hit points. Most of the time, you don't even have to apply status effects to them, since most of these effects come with hit points, and a Minion dies if he takes damage.One way to make minions more formidable, is to significantly increase their defense stats such that there's only a 20% chance (or less) of hitting them, while still only having one or a low number of hit points. They could be the guards which actively obstruct the players from entering a room.
Of course there are exceptional cases. Powers that deal damage without an attack mean automatic death for a Minion. One thing I have considered is to give Minions a Resist X value depending on level or tier, basically giving them a damage threshold. Most attacks that deal automatic damage deal only very little of it. You still don't need to track anything, you just always have to roll damage for a Minion. But that happens most of the time anyway...