While Landorus-Incarnate is outclassed by its alternate form, Landorus-Therian, Lando-I is still the 2nd-best Ground-type attacker, just behind Lando-T. More importantly, the Landorus formes (should) share Candies, which means you can raid Lando-I to amass Landorus Candies for powering up Landorus-Therian.
The Landorus-I duo is easy. While Landorus isn't quite as squishy as Rayquaza, it shares the same 4x weakness to Ice, meaning Mamoswine and co. will make quick work of it.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
Porygon-Z must have Hidden Power Ice to be duo-viable against Landorus. If it doesn't have HP Ice, it's not good enough, even at Level 40.
The difficulty of Rock Slide depends on what you're using. If you're using primarily Glaceon or Weavile, or other Ice-types, Rock Slide will be quite a bit more difficult than Earth Power or Outrage. If you're using primarily Mamoswine, Rock Slide will be easier than Earth Power to go against.
With no dodging, Focus Blast is much harder than the rest of Landorus' moves. However, Focus Blast is a 1-Bar move, meaning it comes out infrequently and predictably, and its damage window is very late, making it easy to dodge. Dodging Focus Blasts will significantly improve the performance of Mamoswine, Glaceon, and Weavile in this fight.