I do not know the answer but judging by the battle reports I receive the number of soul shards generated depends on the level of the troops killed/wounded, the level of the attacking troops is irrelevant.

Killing 800 T1s gets you one soul shard.

That figure came down to 59 per shard when killing an assortment of T4, T5, T6 and T7.

(edit: 400 T2  and 200 T3)

Hi Jarls! To receive 1 Soul Shard you have to kill the following number of troops of the corresponding tiers: 

- tier 1: 1000;
- tier 2: 500;
- tier 3: 250;
- tier 4: 143;
- tier 5: 84;
- tier 6: 56;
- tier 7: 34;
- tier 8: 13. 

Have fun😇

and I need two hundred and seventy-five thousand soul shards to get Freyr past level two - ..... just saying   :-) ............ (need lots of patience in this game).