no actually exoticat is right, i have a simple formula, keep training one thing only like str or def (strength is more beneficial however as it helps in killing bosses faster, and defense is normally not working greatly on bosses as far as i have seen, defense is for player vs. player fights.)

So keep training str, and never train def more than 50 percent of your str. This way you will level up slowly and take out maximum from any boss. And if you consider that you are now not getting good drops from a particular bosses, train def there, to have a quick boost in levels.

It's just a suggestion as per my experience. I am not forcing anyone to apply or go with my strategy.