Partial cover is provided by level 1 hexes or depth 1 water: no one is getting partial cover in this scenario.
Terrain along the LOS between two ’Mechs intervenes if:
- The level of the terrain or feature is equal to or higher than the level of both ’Mechs; or
- The level of the terrain or feature is equal to or higher than the attacker’s level and adjacent to it
(regardless of the level the target is on); or
- The level of the terrain or feature is equal to or higher than the target’s level and adjacent to it
(regardless of the level the attacker is on).
The only exception to this is if the two mechs are adjacent, because adjacent mechs always have LOS to each other (well, with a couple of relatively rare exceptions).
(If you're a long-time player, note that earlier editions had the same rule, but the terrain needed to be higher, not equal to or higher. This was changed in TW.)