I personally run Sonnia a fair bit, she is me go to master when playing guild, and I have found that the witchling stalkers alone make for excellent melee monsters. At five stones with MI 6 and 2/4/5 dmg profile (3/5/6 when you add in burning) they are pretty devestating, add in their explosive demise setting things on fire and they don't even have to live long enough to swing their swords to be effective.
Tthe key is to use them like grenades. When you see something heading towards sonnia, or something else you want to protect, have a witchling stalker either charge or (if you don't have the range) double walk into melee with the advancing foe. It's MI 6 makes disengaging difficult, and if they manage to kill it they will burn up most of their activation doing it and end up on fire. Then sonnia just uses confiscated lore for a cast of nine, ignores cover and LOS and fries whatever was approaching. Even if the stalker doesn't die I'll usually still shoot into melee with sonia because if I end up hitting my stalker I can usually cheat it's defense flip low and then use it as a focal point for blast dmg, or alternatively hopikins rapid fires on it and with a minum dmg of 4 on a burning target few things will survive that. Add in sonnia's reincarnation aura to replenish your supply of witchling "grenades" and it's a devestating combination.
As a side note I'm also a big fan of bringing papa loco with sonnia for the + to dmg flips as it makes it soo much easier for her to pop out the blast markers.