How can I destroy portal shield?

Can I destroy the shield on a portal before all resonators?

If yes, what is the best way to do that? Launch XMP burster on the location of portal? Or just attack resonators and the shield will be destroyed?

From what I've seen, the shields get destroyed intermittently while attacking a portal. I've had shields destroyed while resonators were still on the portal.

Also, it seems like the last shield doesn't get destroyed until the last resonator is destroyed.

As for purposely targeting shields, I have found no way of doing that.

In my experience the shields have a chance to fall every time an XMP is fired that damages at least one resonator. I think I read somewhere in a tactics guide that you can use lots of low-level XMPs from outside the portal's range to "chip away" at one of the resonators, giving the shields each time a small chance to fall.

See: http://www.reddit.com/r/Ingress/comments/1fehcy/ "I've been seeing a couple of L1's hitting my L6 portals every time they walk by and hack. While it isn't doing damage, I am having to replace about 3 shields out of about 80 total shields a day."

It's possible to destroy the shields before destroying the resos. Just stand exactly over a portal or else inside a shield which surrounds the portal and fire an ultra strike with 20% power. The higher level ultra strike the more rare shields will get destroyed. Try it. Or else try power fluctuations. Eg-first fire a high level xmp then low level theen high then low.....keep doing this till shield gets destroyed. The shield gets destroyed due to the power fluctuations.