Curse surge should be every 5 minutes after killing the rare.

You killed the rare? Wait 5 minutes until next surge. No matter if rare died after 1 minute or 3 hours. After it's killed, the new one should start after 5 minutes.

Having an achievment that demands 300 surges with a downtime of 45 minutes is ridiculous.

If blizz don't want to change ir, then reduce the achievment to 100 or 150 surges