User blog comment:Crimson and Darkness/Shin DxD 4 discussion/@comment-78.10.210.206-20200220175352/@comment-28884330-20200220182703

It's not like only the heroes can become more powerful, so this is an inaccurate analysis. The ExE people can also become more powerful, or organize their forces more effectively, or develop additional weapons or forces during that time. There's also the chance that they have other things they are dealing with as well, DxD is not the only world or faction they are fighting. FxF is a world already conquered by them so there's the chance they might be warring against even more worlds currently, not to count the war they have against Resetoras' faction. You are ignoring a lot of factors.

Also, you're acting under the assumption that they have total knowledge of the DxD world despite such a thing not being mentioned, just because us as the audience have such knowledge. As far as they know, Great Red is not necessarily the strongest being in the DxD world, so attacking carelessly up front would be tactically unsound.