These are some personal notes, not intended to be a guide.

• Landslide has an innate 5% CC, 5% SC.
  These can be buffed but the ability usually benefits most from flat damage bonuses.

• Landslide gains 25% extra damage per melee weapon combo multiplier.
  This is factored in separately from the built in Landslide combo meter.

• Uses: Base Damage x (1 + Damage Mods) x (1 + Elemental Mods) x (1 + Ability Strength)
  For maximum benefit you want to spread stats between these categories rather than focusing into one.
  So aim for a mix of +Damage, +Elemental (inc. +Impact) and anything that helps build or sustain melee Combo.

• Weapon choice does not matter beyond the available mods, riven disposition and being able to sustain melee combo.
  I don't think attack speed affects Landslide but needs testing.

I was also interested to discover that Landslide fully benefits from subsumed abilities that otherwise only buff weapons, such as [Mirage]'s Eclipse.

Roar might proc twice and multiply when simultaneously buffing weapon damage and ability strength. EDIT: Under very brief testing vs Eclipse, Roar does proc twice and is more reliable but is underwhelming by comparison.