I hope you see the problem though, if by the rules a mob of 100 peasants won't even inconvenience a 15th-level Fighter, but by your DM ruling the high-level Fighter stands no chance.
Fighting large numbers of "mooks" isn't exactly an off-the-wall scenario the game-designers couldn't expect, but here we are with the rules saying "no prob" and a DM saying "no chance".
I see levels as an abstraction.. kind of like hit points.
In the same way I'd narrate the 15th level fighter being overpowered by 100 villagers, I'd also narrate the same 15th level fighter slaughtering 10 of them (without rolling any dice because it's pointless).
I think the 4e combat system is designed to represent situations where the outcome is in some doubt. And I think it does that well, especially allowing weaker foes to be represented by level-appropriate minions, and stronger foes to be represented, in mechanical terms, by level-appropriate elites and solos.
I don't think levels are a concrete value intrinsic to NPCs; they are abstract and relative to the PCs.
D&D is not a simulation.
So while the PCs are absolutely 9th level (although their level is still an abstract value), I might decide that a group of opponents are nothing more absolute than 'low heroic tier' (a vague yet absolute statement of ability).
But because those opponents must have a level in order to generate combat staistics, they might be represented as 10th level minions - roughly equivalent to low heroic tier standard monsters.
If I had decided the enemies were 'high heroic tier', like the PCs, then I would just give them a level somewhere around the PCs level (9th).
Likewise, if I deem an NPC's power level to be 'mid paragon tier' (in absolute-yet-abstract terms), then in a fight against 9th level PCs I could represent this NPC as a 10th level elite. This is the same NPC who might be represented as a 15th level standard monster if he were to fight the PCs in a few levels' time.
Hell, the same guy might be a 6th level solo against a party of roughly that level. My point is that the values used in the combat system are designed to be abstract representations and they should always be relative to the PCs (who are the only ones who need an absolute level value at all times).
The statistics don't exist outside of combat between the PCs and NPCs. All that matters otherwise is a general idea of the NPCs' prowess, which needn't be any more detailed than a rough indication of which end of a particular tier (heroic-paragon-epic).