You'd be better off posting some details about the problem - error messages, machine specs and such.
I'm assuming this is the throwback game, as I have no bloody idea what an alchemical hydra is.
Either way, if this is happening consistently and only on that one NPC, then it's probably an issue with your game cache and you should follow the instructions to rebuild it here: https://support.runescape.com/hc/en-gb/articles/360001441169-How-to-clear-your-cache-
Make sure you let it rebuild again before playing - on NXT you'd sit in the lobby for a while, but I don't know how you'd see your cache size on the old client.
If this is not happening consistently for that NPC, or is happening in other areas, then it's probably a networking problem. Diagnose your LAN for reliability and signal, and if that doesn't help, try a call to your ISP.