Chronoweave Instance

I think that's what it's called. I went into the Chronomage Zone and started to do the quests there. This is my first time running through these. I was doing great until I entered Miragul's & Desert of Ro instances. Are the top bosses supposed to be immortal? I spent over an hour zapping that Ice Lord guy on the throne with my merc & pet plus all the others assassins as a lvl 125 Nec and could only drop his health from 100% to 97%? He vamps his little minions to stay healthy. I even set my merc & pet to a dedicated attack on the guy while I ran around hitting his ice skellies, but couldn't get him to drop any lower. In Ro, a Sand Giant attacks along with a tarantula that both bob between being dead (zero life without falling) and full life. All the NPC's in that zone are helping (Ranger, Beastlord, Mage, etc) but nothing drops these two below 97%. Half my spells won't work when I want them to because I get "target not alive" until their life bar blinks on up again. I finally used escape potions to get out of those two zones. I took down the bosses in the other portals. Is this a bug? Or is it the way it's supposed to be? In these two zones, the targets con yellow^^^^ to me. With my pets, I can usually take them down, but not if they are immortal like this!

Several of the Chronoweave bosses have specific mechanics to kill them plus as time has gone on the damage output of our characters has outstripped the HP/Levels of them making sticking to the mechanics slightly tricky

Terrorantula and Ancient Cyclops - If you have done the PQ you may have seen the script being used (and shouted about). You have to kill Madmen, then Mummies in the graveyards.
At this point it is best to head to the entrance as the AC will spawn on you and the Terrorantula will spawn shortly after. The Script Needs the NPC Ranger to Agro the Spider and then Lead it to the AC, Killing the Ranger (Ranger Down shout emote) once the Ranger is dead the spider then the AC can be killed.
If for some reason the Spider is agro'd before it kills the ranger you need to either clear that agro (FD works in solo) or position Spider/AC so their forward directional AoE's will kill the ranger.

Miragul's Madness.
There is a quest through the zone to make a potion that is needed on the final boss, you harvest a Bottle from the room with the Holgresh in, a feather from right clicking on one of the non-agro birds, a hair after speaking to the barbarian npc and a lore item from the ulthork named.
Right clicking the potion bottle combines all 4 into an item that you need to use when you get the message during the final fight that it has gone into a rage and you need something to calm it down.
(Also as a side note, don't ignore the message that something is slipping away with the treasure as there is another boss spawned in the large room outside where you are)

Although you didn't mention it, the Magus in Nagafen's Lair also has a painful mechanic in that you need to stop doing damage (You, your pet and your merc) when he emotes that is is drawing power, as it is stored and reflected back after a short period.