In general summoning schools do not receive much love in this edition.
Necromancers are probably the weakest wizard specialization on 5e. Sad considering how necromancers are amazing on 2e and on 3.5e...
I an trying to solo BG2 as a Necromancer, on Legacy of Bhaal difficulty and OHK spells and summons are saving my life since monsters has so much HP and always hit. If you put "lower resist -> lower resist -> Great Malison" in a Spell Trigger and then, cast Finger of Death in the enemy, it kinda forces the enemy to make the save vs spell at -8 disadvantage (-2 from necro specialization, - 2 from Finger of Death spell, -4 from Greater Malison). Since the Greater Malison is casted on the trigger AFTER the lower resistance, if the necromancer is lv 20, they have 95% chance of sticking(65% = Base MR - 30*2 from LR). Firkraag has a 65% chance of failing a save VS finger of Death if the Malison is on and 45% if is off, due his saves.
Combined it is around 60,8% chance of OHKilling Firkraag as a lv 20 necromancer. I an lv 18 at moment(imported a character only for Legacy of Bhaal), so my chances are smaller. But 60% chances of OHKilling a dragon is not that bad if you are a lv 20 caster.
Here is a screenshot of I killing him with Finger of Death. He resisted my first FoD but failed vs the second. And the summons are more to cause distraction, not to fight him. Dragons on BG2 has a lot of nasty spell effects, they aren't giant flying lizards.
I also tried to face him earlier BUT i could't even reach him without some HLA. Adamantine golems are too dan powerful and on Legacy of Bhaal, there are a lot of then protecting his dungeon. When i soloed on Insane, Adamanting Golems was far rarer.