New attempt at explaining this right, since I'm 90% sure that joela and I don't actually disagree, it's just an apparent disagreement because I'm glossing things over:
For the GM
E6 isn't just a change for the players: Monsters are presented differently than in d20. Just as level 6 parties in D&D aren’t expected to tangle with monsters higher than CR 10, the mighty monsters of E6 require special consideration for presentation in-game. E6 characters aren't intended to go up against high-level D&D threats under the same circumstances as high-level D&D characters; those creatures, if they are defeatable at all, require the kind of resources and planning far beyond the typical D&D encounter.
In terms of raw rules, CR 7-10 monsters are an excellent guide for what E6 characters can handle. Beyond that, a DM should take CR 7-10 monsters and use feats (and to a lesser extent templates) to advance them. Hit die or class-based advancement beyond CR 10, or base monsters above CR 10 should generally be avoided as straight-up fights.
Of course, not every monstrous encounter is a straight-up fight. For example, insane horrors from another age might be a reason to run, and there is little a character could do in the face of an angry Titan. But these situations don’t call for direct confrontation, except with some special resource or amazing circumstance. Perhaps, in a special ritual with the presence of 20 mages, a Titan can be bound to the mortal realm (lowering its stats to an Aspect of Kord), with whom the players can do battle. Again, that's far from a straight-up fight with a CR 20 creature, but we can console ourselves with the fact that this is an awesome encounter.