A lot of people always mention that merkava's 3c into 214a/b is safe, but I've been in the lab testing it and if you shield the 3c you can run under the projectile for a full combo punish. Also, if he makes an attempt at delaying it so his body is closer to the ground to make it less punishable you can punish the delayed fireball (beat it on start up rather) with 5b into a full combo.