Possible Clues:

• The brute makes the air poisonous
• At first the last of 30 across needs the mixture of air to be poisonous
• 27 across is over the poisonous air
• The poor brute makes a mixture of air so poisonous
• The brute is deadly on the air
• How poisonous for the brute to get the mixture of air
• Amazon muscle relaxant: Var.
• Poison the brute with a mixture of air
• If it's poison, the brute may need a mixture of air with it
• With this poison the poor brute needs some air