Well, having minions durring the fight helps with action economy, and having items or spells that grant immunity to certain effects helps with the save or suck abilities and spells.
If you feel that giving him immunity to a player's encounter ending ability/spell is too much, you might have luck with having a tough 'mini boss fight' before the big bad, and have some reason why they can not rest for eight hours after the mini boss. Sometimes the mini boss fight will end up being the harder fight, however, if the party knows that the big bad is next, since they want to save everything for the final fight. A way around this is tricking the players into believing that the mini boss is the big bad.
