The first phase only uses melee and thus can easily be safespotted. There is no 'fix' for this issue, unless we use a new map or change the corporeal beast's combat mechanic.
The dark core would be excellent at preventing safespotting, it would make corp a ton harder to solo as well which is nice. First phase could be made to recoil equal damage from ranged/magic instead of protecting from it, that would quite literally nullify safespotting. Skeletons could be used to fill in the smaller safespots, a rockslide could cover the large one.
That's all up to you though, I just think this makes corp a lot easier than it should be.