the best I've ever seen Broner fight above 135 was probably against John Molina. In order to beat Mikey he should box like he did there. NO standing on ropes, sharpshooting, etc... but constant lateral movement and working from the outside actively.
Mikey Garcia is a classic boxer-puncher imo, the same style as Arguello was (others said this alrady but wel... he really is). Good all around fighter including good power, not a speedster not exactly a tactical mastermind or a defensive wizard but has rock solid fundamentals and knows can fight backwards and forward too, economical, takes little steps rather than rushing, doesn't like to punch in a fly, cerebral...
to beat him Borner needs to build on his speed and athleticism advantage not so much on his size and possible power advantage cause I think he generally needs to avoid mid distance or inside exchanges with the more compact and very dangerous Mikey.