I'll describe in more details how do I get to invisible enemies and invisible effects/closed gates.
I went to the second area, did part of the job (opened one of the gates or defeated one of the enemies) and then went out of balance, returning to the necromancer. Then if you proceed to the second area you'll get an invisible enemy or a closed gate (which is actually open, you can go through it). Though enemy seems to attack me only once/twice, it was relatively safe.
Loved the fight with Sol (Lun is similar I guess?). I panicked several times during a fight, keeping a constant look at the balance and where the enemy is now, but eventually I've beaten it.