I don't have yet Walking Bomb to test, but I guess these explosions don't play well with friendly fire active, do they?
Now this is a good point, a dangerous good point. On lower difficulties you can turn FF off. I haven't tried Nightmare - but I hear supposedly you can turn it off on that as well?
If this is true, then you have no problem. If you have it on for a bit more of a challenge and or realism;
PLEASE USE THE TACTICAL CAMERA!
Do not let your tank and or other melees jump head first in during the intile cluster of Bomb explosions, don't do it. Friends don't let friends explode.
As for my -current- set up.
1. Barrier
(This is required, on all mages. This is not negotiable in my mind. Due to there only being -1- healing spell in the game, it being a focus and it being for a DIFFERENT specialization than our own, you NEED Barrier.)
2. Immolate
(I love this spell. The raw damage is lovely, burning and fear stack well with Necromancy, low CD, low mana cost. Love it.)
3. Horror
(I've come back around on Horror, but only slightly. I still think that if you DON'T upgrade it, it's more or less useless. Even with the passive on the other side of our tree to help stack a debuff, without Horror's own upgrade for debuffing, it falls off to me and is a point better spent elsewhere. That being said, I have it's upgrade so I'll keep it - for now)
4. Walking Bomb
(I don't think I need to tell fellow Necromancers, this is more or less why we do damage in large groups. I find it much more useful in DAI than it was in Origins, which is good because the idea of the skill in and of itself is awesome.)
5. Spirit Mark
(Secondary bread and butter skill for us, I can't explain how useful this is beyond the fact I can turn Giants against each other and or against other enemies. Giants. Yeah.)
6. Fade Step
(Obligatory "I royally screwed that one, bye" skill)
7. Winter's Grasp/Chain Lightning
(So this is dependent on personal preference. I believe in having more than one elemental skill, mainly just for variety coverage and for dispelling those pesky magic barriers around the world. I still don't like chain lightning's reliance on the upgrade to be worth it in my opinion, and winter's grasp has the added benefit of chilling. But it's up to you.)
8. Focus Skill/Fire Mine
(Another choice, so I'm one of the few people that I've seen so far on the forums that actually considers the focus abilities. The main one we get during the storyline is... well it's pretty damned strong if you have Perks to get to level 2 or 3. That being said, our specific focus, Haste - can be really useful as well for re-positioning and the like. As for Fire Mine? Look at the damage. That's all I have to say on that.)
This is just a very early skill build, the one I'm using at the moment. Thanks to how cheap re-speccing is it will no doubt change over my play. I also have a specific team set up for this build.
Tank - Quite honestly on that front, I prefer Cassandra to Blackwall - a choice I'm sure many will disgree with. I have her set up very Tank-like, obviously going heavy into Sword and Shield, getting a lot of her Templar tree, and going a bit into Vanguard.
Another Ranged - usually another mage, honestly solely for the stacked Barriers. I prefer Dorian on this front. Usually just for the doubling up on Spirit Marking, but I will use Vivi more or less solely for Resurgence if I royally muck up. Solas is a nice synergy as well what with his "AOE ALL THE THINGS" mentality. So in short - any mage I feel like.
DPS Mastery - A DPS filler, almost always used with Iron Bull. If not alone for him being awesome, the fact is when you mix a Reaver and a Necromancer - everything dies. It's just scary. And with a second mage always being able to stack Barrier, if he spams Dragon's Fury too much on his own, I simply switch to him, Devour, switch back and Barrier. It's all good. I do at times substitute Sera, simply for the ranged benefit of the secondary mage always catching the three of us ranged chars with Barrier, and since Cassandra always has guard, good to go. Also, Thousand Cuts? Yeah.