Yes
The Pokemon Aggron is the final evolution of the Pokemon Lairon, which evolves at level 42. It is not able to learn the move Earthquake through leveling, but it can be taught Earthquake using TM26 at any time.
Nidorina cannot learn Earthquake by any method.
TM 26: Earthquake. No doubt about it.It hits hard and many Pokemon can learn it.
Earthquake can OHKO, yes, but it's not specifically an OHKO move. It is, however, a powerful move, so it's a good move to teach a Ground-type Pokemon or a Pokemon with a high physical attack that can learn it.
Most Pokemon can learn that move think of it this way Pokemon that have hands can learn it some Pokemon learn it from the move tutors however some can learn it via egg move only.
Grotle is not able to learn the move Earthquake by leveling or by TMs. It's evolved form Torterra is able to learn Earthquake at level 32.
Rampardos can learn the move Earthquake though not by leveling up. You can teach Earthquake to Rampardos by using the TM 26 on it.
The move Earthquake make the ground rumble and every other Pokemon's hp falls also if in a double battle your other Pokemon will be affected too.
Then that Pokemon cannot learn it. Only starter Pokemon can learn the ultimate move. No other Pokemon can learn them.
Any Pokemon that can learn a electric move
Torterra either learns it level 32 or learns it through TM 26. If your Torterra is higher than level 32, then you should go to Electivires Link shop and get Torterra to remember it. Remembering a move is for free by the way.
In order to learn a new move, the Pokemon must forget moves first. Then, it has to level up and it will learn the move it should have already learned.