It's been a while since I read the other chapters, but I don't recall El-Domo giving a shit about Tina. If all he sees in her is a sacrificial pawn, then it makes sense that he would have her attempt a suicidal attack even if the chances are practically zero. El-Domo's probably thinks that if Tina somehow manages to stab Gareus with a poisoned knife, then Garron has a better chance in the fight even if it's only 0.00001% better. And if Tina fails, then El-Domo would have thought he lost nothing by trying.