You can't directly affect what runes your troops get each battle. Creatures with "Runic Knowledge" always start with a couple of predetermined runes, Runic Magic talent and some artifacts add random runes on top of that at the start of the battle and Runic Word adds a random selection of runes.
It's not entirely random in that it tends to add the runes pretty evenly (so if Runic Word adds 4 runes, you usually get 1 of each type and a 4th randomly), but there is no way of choosing directly.
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