OneTaq® DNA Polymerase (NEB# M0480, NEB# M0481) has been developed with a GC Buffer and High GC Enhancer that aid amplifications of difficult templates. Q5® High-Fidelity DNA Polymerase is supplied with buffers and High GC Enhancer, and has been optimized for robust amplification, regardless of GC content. You could also try 7-deaza-dGTP in conjuction with normal dGTP in order to destabilize difficult structures. Note: 7-deaza-dGTP attenuates the signal of ethidium bromide staining. OneTaq® and Q5® are registered trademarks of New England Biolabs, Inc.