Very carefully
Level 57 (Diamond/Pearl/Platinum/HeartGold/SoulSilver) Level 77 (Black/White/Black 2/White 2)
nidoking will learn megahorn at level 43....thats the way it is...ok men?
Heracross learns megahorn at level 55
Megahorn isn't a TM. You'll need to level up certain Pokemon to get the move. Such as; Nidoking, Rapidash, Rhyhorn, Rhydon, Goldeen, Seaking, Heracross, Rhyperior, Samurott, Scolipede, Sawsbuck, and Bouffalant.
Level 53.
rhyperior learns rock wrecker at level 61
Rhyperior can learn very powerful moves like Mega Horn, Rock Wrecker, and Horn Drill (a 1-hit KO move).
It learns Thrash at level 22 and Megahorn at level 43.
It learns Rock Wreaker when it is level 86.
A good hold item for rhyperior would be hard stone, soft sand, quick claw, leftovers, brightpwder, scope lens, dire hit, any of those would be great or even focus band or focus sash.
1 stomp 1 horn attack 1 furry attack 1 tail whip 9 stomp 13 furry attack 21 scary face 25 rock blast 33 take down 37 horn drill 42 hammer arm 45 ston edge 49 earthquake 57 megahorn 61 rock wrecker
Rhyhorn indeed evolves to Rhydon at level 42. Then, you can evolve Rhydon into Rhyperior (number 464 in the National Pokédex) if you trade it while holding a protector.You might want to delay the evolution of Rhyhorn, so that it gets certain attacks earlier. For example Megahorn on level 67 (Rhyhorn) instead of level 77 (Rhydon and Rhyperior).