Well, if you have no cold enchantments, damage output of Niacs DoT had to be poor
Additionally, trolls are weak to acid and fire, so advantage of FW in this fight was even bigger.
Lev 5 DoTs have unique mechanic - they are stacking with themselves up to 3 times. First stack is doing poor damage. Second is pretty nice. But third stack is extreme - base damage is 3d6 + 3 x your level, or 3d6 +27 in your case every 2 seconds. For comparison, FW base damage is 2d6 + 9 every 2 seconds for you.
My advice is to put points only into damage enchantments for all 4 elemental lines, dropping single points into crit chance and crit damage lines when you have some spare points
and maybe a single point into force line (+20% for first tier on disintegrate is worth it IMO). This lets you use full repertoire of your spells, both lev 5 DoTs, firewall and acid rain. You can then choose right tool for your job, as every wizard should (and yes, I know that right tool is Circle of Death and Wail of the Banshee in 95% of cases
).