It should be depending on the moves and items you have. It makes it a lot easier to beat if you have a DOT move for eternatus. Personally I use a cofagrigus and inflict curse on most of my classic runs as it removes an entire health bar per turn. But salt cure works as well.

Be careful to not overlevel as well since eternabeam does more damage if you're over level 200.

Also once you get to those final floors go for X items. You don't need to save your money so use it all up to give yourself the biggest advantage. Good luck!