Use the Flame Tounge Totems to give the minions you receive from Gothik the Harvester attack so you can use them to kill Gothik the Harvester’s minions, don’t worry too much about the damage those minions do to you each turn.

Beating Gothik the Berserker in Heroic mode: http://wp.me/p1SfP0-1Lt
Beating The Four Horsemen in Heroic mode: http://wp.me/p1SfP0-1Ls
All Naxxramas Guides: http://wp.me/P1SfP0-1L4

It will be important to focus on board control rather than damage to Gothik the Harvester. You also need to be careful when you face the Abomination. Having Nerubian Eggs out against the Abomination is very good. Kill the Abomination using minions that would die to the 2 damage Deathrattle of the Abomination anyway.

Use the Hex on targets like the Unrelenting Rider or other big targets, especially those that have a good Deathrattle or active ability.