As the game progresses and you accumulate more and more ‘perfect’ items and full builds, difficulty decreases by much, fully equipped team will not have any problem on max difficulty. More than that, its not hard to get ‘duo’ achievement on max difficulty.
But even with full builds game is still more or less challenging at max difficulty(unless you resort to cheap tactics like spirit guides), and that what matters the most. The purpose of the game of this genre is to keep you equally entertained at level 10 and at level 1000.
As a tip for acquiring achievement for beginner party – pass difficult waves, try to beat act 1, and always use potions before difficult battles, +quickness is great, and regeneration\damage protection would be of great use for your tanks.