Snikrot.
Becuz green iz best!
Orks are tough. As in, a gun which cuts humans in half warms them slightly. It takes rapidfire RPGs, molecular shuriken, swarms of ultra strong scythe bugs, or ANTI ARMOR WEAPONRY just to kill them.
Orks are also strong. They can have tons of metal bits grafted on to their skin, as well as a generator on their back, and still walk around and fight.
There's also the matter of technology. Cloaking? Kommando speshulty. Dakka? Ork speshulty. Big boom? Ork speshulty! Moar dakka? Youz getz da picture.
While Ajax's shield may be effective against swords and bows, a single stikbom could tear it apart, if not Ajax himself. A shoota could just punch right through. If Snikrot had a choppa, MAYBE Ajax could win. But even choppa Kommandos usually carry a pistol, and I've never seen a Kommando without some kind of explosive device. And let's not even go there with shotguns and flamers. As for Ajax's weaponry, a spear is almost completely useless against an ork, as they don't feel pain, and have skin comparable to armor. And they wear armor made from the skin of Squigs, which are nonsentient cousins of orkkind and thus have similarly tough skin.
And then there's the fact that Kommando's are some of the most kunnin' orks ever. Admittedly, that's not saying much, but orks are very good at fightin' an' winnin'. Snikrot would likely just shoot Ajax in the back or plant mines or something, but if things got dicey, he'd probably either recloak and attack from behind or do something crazy like use his own equipment as improvised explosives.
So there's my argument as to why Ajax is completely, utterly, screwed.