Oh I'll tell you how you do that. It's very simple. You get people skilled for this exact task ! Those skills are acquired in universities(preferably good ones) where professors teach courses like "Formal Languages and Automata" or "Compiler theory".Actually Patrick Michaud, author of the PGE and NQP-rx grammar engines was a professor of Computer Science himself, and is sufficiently qualified. Still he's just one man with business to run and a family to care of.
I myself took a course called "Formal Languages and Automata", and while it was very interesting, only about 20% of what was taught there was relevant for implementing a grammar engine for Perl 6. None of the students was eager to implement a grammar engine themselves. Building one that's usable for a real world task like parsing Perl 6 is a much larger project than a typical academic exercise, and most people from academia are not really interested in such chores.
Anyway, if its "very simple" for you, please do organize a bunch of motivated CS students that help us improve the grammar engine. Until you do I don't believe it's very simple. If you do, I'll be very happy to humiliate myself here publicly and admit that I suck at attrackting developers, and you rock. I don't care as long as we get new contributors.
If you can't get real compiler people to use Perl6 and help with it, the average open-source rookie won't be able to deal with this
Most of the tasks while writing a compiler actually don't require much knowledge of deep compiler theory. What blocks people from contributing more to the guts of Rakudo is that you need to be quite familiar both with Parrot and Rakudo internals.
Also writing a compiler is not something that you either can or can't - you can learn it if you are motivated.