CP of 716, HP is 306. Battle with it vs one with higher CP?

TLDR: Chansey bad for attack. HP is not a good value to rank attackers on.

Each Pokemon has different stats in their attack, defense and health (HP). I am not talking about IV here (that's a whole different story, read for example here ), but about the base stats for each Pokemon. A Chansey has a very high base stat for HP (currently only outranked by its evolution, Blissey), but a very low attack. Hence it will last long in battle, but do not a lot of damage. Therefor it might not be an ideal choice to use it for battling, but it's better for defending gyms for example!

Very highover, a higher CP means better stats, meaning more damage. There are exceptions, like Chansey, Blissey, Slaking and maybe Aggron. And I probably forget a few less favorable, high CP Pokemon. Also, type-effectiveness is a very big thing! If you are taking down a Rayquaza, a Dragon-Flying Pokemon, ice is double super-effective against it and does 256% damage! Ghost is doing only 100% damage, so a Mamoswine or Glaceon (Ice Eevee) with the right moves might do a lot more damage than a higher CP Gengar!

These base stats are not visible in-game, but can be found on the internet. I do like Gamepress myself, because they have a (sometimes out of date) description with what moves to use for each Pokemon. Here is a link to the Chansey from the question: here.

edit: I just saw what Gamepress had to write about PVE with Chansey. This also says enough :)