Fake Out would be, 100% of the time actually. Mat Block has 0 priority, whereas Fake Out has +2 priority, guaranteeing it moves before Mat Block regardless of the Pokemon's Speed. Taunt has 0 priority like Mat Block, so it depends on which Pokemon is faster as usual. Greninja has a high Speed stat, so using a Taunt Pokemon with a higher Speed stat would be ideal. An alternative would be the use of the ability Prankster, which gives priority to status moves like Taunt. Use a Prankster Pokemon with Taunt like Sableye to guarantee you move before your friend's Greninja and prevent it from using Mat Block. Of course, doing this on the Breloom is a better option for the reason stated above.