Post 'em if you have 'em ... not necessarily tricks, but ways to deal with situations more quickly and efficiently, miscellaneous tips, etc. I'll post 2 here.
When a battle with a wraith drags on long enough, a wraith will enter invisibility mode. There is a short animation, and a message appears in the log. What many people don't realize is the wraith is invincible during this time. Spamming holy light or fire spells will not do anything. Instead, everybody should take this time to recover and buff/debuff as needed. If the wraith charges up his attack, that is time to unleash your strongest attacks and spells (holy light is not needed). If enough damage is done, invisibility will break. Sometimes he will not do his strong attack, but you may notice the same little sound and animation that he did when he went invisible, with no log message. That means he came out of invisibility.
In terms of breaking his attack, naturally an ark can just turn break him, but without an ark it generally takes about 5k dmg to break the fmq wraiths, 8-10k to break a wraith in a gr26 quest, and around 25k to break abaddon in fmq.
Opening duke/general gigas
I think we all know the basic idea, but if you don't: When it looks like they are charging up an attack, you will see a bluish glow on one of the 4 eyes at random. Striking that eye will do damage to him, and break his attack. When he loses around 20% of his health, he "opens up" to a regular gigas-type enemy. There is very little danger when they are in their closed-up "turkey" appearance, but still why drag out this part of the fight when you don't have to? Based on your party, there are very efficient means to getting this part over with. Everything below assumes a party of equal level... of course a GR30 axe would be more effective than a GR20 mage, so just assume that all are at an equal level with the info below.
The hierarchy of damage that can be dealt to the glowing eyes, from strongest to weakest, is
mage (but, a well developed gr24 mage could overpower well developed gr30 axe/ls)
Also, never cast slow or use anchor blade in this phase. His attacks will never happen anyway, so save this for when he opens up. Based on the members of the party, split up so there are 2 people watching 2 eyes. The ideal situation is having 2 mages. 1 focuses on eyes A and B, and the other on C and D. The basic principle is to have your 2 strongest team members each focus on 2 eyes and the rest of the team just handles appropriate debuff, recovery if needed, or just sit back and watch, ready to strike when he opens up.
In quests with a turkey boss, like JD and STT, 1 person can easily "tank" ... which means do the damage to them and even get them opened up by the time the rest of the team clears the battlefield. As a mage, stand where you can see 3 eyes, with EG ready to go, targeting the eye you can't see. If you see the glow, shift target, otherwise unload. A melee can do something similar, except they need to move to the target. Still, stand where you can see 3 of the eyes.