I'd stick solely with standard Elementals to start. Start with how they take damage, then how they move. If they comment on the low speed, explain the gist of the mechanized infantry rules, while also pointing out the high speed of most Clan Omnis. Tell them about the guns, first what each suit carries, then explain that they fire as a squad, so they shouldn't think of a suit with a Small Laser and SRM-2, it's actually a Point with an SL-5 and an SRM-10. Explain leg asks and swarm attacks, going through an example of each to show what damage suits can do at pointblank range.
If you have access to the old Battle of Twycross scenario book, The Terrible Toads is an excellent scenario for introducing an inexperienced player to Elementals.