Assuming I've got a proper handle on the rules, a Witchling makes a great little target for Sonia's Flame Burst. Run the Witchling so he's within 2" of as many models as possible (in the game I played last night, I had him within 2" of Madame Sybelle, a Rotten Belle and a Mindless Zombie, and only about 2.5" from Nicodem), then lob two Flame Bursts at him. He can choose to fail the resist duel, so the spell automatically hits him. Ideally, cheat fate so that the blast does Severe damage. If you can do that, the first spell will cause 3 damage to the Witchling and anyone the blast templates touch. The second will do the same, and kill the Witchling, causing his Immolating Demise to go off for another 2 damage. If you're lucky, 8 damage to multiple targets in just two activations! Also handy against those pesky Hard to Wound undead...
If I understand correctly, Flame Burst is not a ranged attack spell, and so it makes no difference if the Witchling is in melee or not.