You need to make the brainsucker taller.
If a unit is only 3 voxels high, it's shoulders (=firing origin voxel) are at height 0... and when validating melee range, the LOF will hit the target tile's floor before it hits the unit on that tile... resulting in "no LOF"...
...or something like that... I didn't have the patience to trace all attempts to draw a line... in any case, the brainsucker cannot target the units with its attack.
With standHeight=6 I got massacred pretty badly