Some things I've learned playing a bunch against a local merkava:
A well spaced 3B will stop merkava's flight -> dash attempts and will also beat his dive kicks clean. Worst case scenario is that 3B trades with the flight dash but he just took more damage than you did.
His reversal is easily baited and has no invul when the attack starts, 41236C or 623C beat it easily. If you don't have the meter, 623B will trade, but he just wasted 100 meter for a paltry 200 or so damage.
His backdash is really good but Orie has the tools to stop it between 66C and 236C
Whenever you see him going for the 3C overhead, immediately use 236B, you win either way. You either get out of his pressure if he does the feint (hilariously, if you have CS you can actually cancel the whiff as you get behind him and run up and punish him for it) or just hit him outright.