Are you looking for Damage per Second or just straight damage per hit?

Ashkandor, mageslayer, slayers wraps?

What's that thing that drains mana every 15 seconds and also gives you +damage?  It goes in the consumable slot,  I think it's 2200 gold trinket. 

If you have to use a potion slot then I don't think you can get Ashkandor.  I think you'd go with mage slayer, doomspite grips, and cloak of flames.  That's 24750 gold.  Slayer's wraps give you the chance at the critical, but doomspite are higher damage and also + attack speed, cloak of flames is +30% attack speed.

10-15seconds sounds good.

   I guess it also depends on what your demigods base autoattack damage is.    If you do 100 damage per hit then something that added +75damage would be better than something that added +30% attack speed.   

But if you were doing 250damage a hit then the +30%attack speed item would be better than adding +75damage item.

Pentince, Bite, and groundspike!

all youde need would be those 3 items the favour item would be mards hammer and you get him to have the 3 god strength skills and hed have 780 per hit at lvl 20 and a rate of fire of 1.314 and not to mention the 4x crit from the askandor makes it even better this would practicly make the rook unstoppable

umm..... 

Ashkandor                   18000

Mageslayer                    8000

Wyrmskin Handguards    1500

Sigil                                500

                                  28000

That's probably the best 3 damage items for 28k imho.  Parasite egg is too awkward for my liking.  The other combo with parasite egg is probably more total damage, but we all know life steal is better

Can I now ask the obvious question, why 28K?

Well blarg to you then mr. manners   

Nope, 25k is the starting gold for filthy rich games.