It isn't just a problem with Torax.  Slow can be extremely powerful when shard-boosted on a number of monsters.  Given how difficult it is now to acquire shard power, some may argue that if you can acquire a -8 initiative slow spell, maybe you deserve to beat Torax.  Haste has the same "problem", but is arguably more overpowered because you can always successfully cast it.

I think a good way to "fix" overpowered low-cost boosted spells would be to have varying casting costs:

Slow 1 (requires 0 water shards to cast)

Slow 2 (requires 1 water shard to cast), reduces initiative even further but costs x amount more mana.

Slow 3...(requires 2 water shards to cast), further initiative reduction but costs 2x amount more mana

etc etc