Then, maybe Thunder Wave or another offensive move could go in place of Explosion?
Thunder Wave would be a fine addition if you're running only one attack. I should note though Regeneration over Magic Guard would probably have more use to you.
That sets main purpose anyway is just to come in and set dual screens, hazard damage could be easily countered. He won't ever be staying for long so he can heal in between switches since you risk having a setup thrown on you.
I don't think dual attacks is necessarily good on Rankurusu if you're going to run Light Clay. Half the fun of Rankurusu I find is that thanks to Magic Guard and his already high offense you can throw on Life Orb to take advantage of his perfect coverage in Focus Blast/Shadowball.
How does Rankurusu do against Rohbushin? Seems like the Cell can come in on a resisted Mach/Drain Punch and take the hit fairly well with his bulk. Because Rankurusu is actually even slower than Roh, the Clown's Payback won't do as much damage, so he'll either switch out or Bulk Up and hope he doesn't get a Psychic/Psycho Shock to the face.
All in all Rohpushin's effectiveness against the little pillow highly depends on the set they run on both sides.
If you're facing a full offensive version with STAB you'll always lose 1v1 with Rankurusu. He is simply far too bulky and its naive to think you'll ever OHKO it without a powerful SE attack (eg. STAB Megahorn) even if you have Guts activated.
The only particular time Rohbushin tends to stand a chance is if its against the dual attack TR variety aka Shadowball/Focus Blast w/Recover and only if Rohbushin is Bulked Up or pre-statused with Guts. But be aware Rankurusu can still easily 2HKO with the non-STAB LO Shadowball and can easily use TR to nerf your payback while recovering off damage.
For the record I run a slightly bulkier one compared to DDRMaster's with 172 HP/152 Def/212 S.atk which only ever takes around 38% on average from faster Payback and 75%ish from full power and 10%ish from a non-Iron Fist Mach Punch.