Sonnia's Avatar is a beast if used correctly for three reasons:
1. Blazing Counterspell. Not only can she protect herself and any model within 3' of her she also punishes the caster.
2. Pyre + 2 Pyre commands. If you do this right you can either wound and give burning tokens to a large number of models or you can concentrate this on one model and just about kill anything in the game if it hasn't activated yet. Read what this spell says, you take 3 dmg whenever you touch a pyre for the first time and then 3 dmg whenever you activate touching one. So if you went first with Sonnia and put 2 Pyres on top of something, that 6 Dmg and 2 Burning tokens during your turn and another 6 dmg 2 Burning tokens whenever it activates where if it survives it still has to take more dmg or become slow to get rid of the tokens.
3. And then there is Light Em Up + Searing Wave. An excellent way to spread dmg around. And if you use it on something which you have already burned to a crisp with your pyres you could easily be doing a straight 2/3/4 dmg flip +3 dmg just from the burning tokens you put on it that round, so a minimum of 5 dmg if you wanted. (Only groups I wouldn't use this on are Rasputina and the Friekorps)
So to add up, thats 6 Dmg 2 Burning tokens from Pyre, a 3rd Burning token from Light em up, then 5-7 dmg from Searing wave, then another 6 Dmg and 2 burning tokens when they activate for a total of 17-19 Dmg and 5 burning tokens (6 if you got searing wave's trigger off) if they somehow manage to survive they are at the least going to be slow unless they can just eat the dmg from the burning tokens.
You just got to time it right and make sure you can cheat over their df with your cast on searing wave.