I'd use rules provided in the Dungeon Master's Guide, page 42.
Being the fire log drop a trick you could perform many times during an encounter, and something that does not consume standard actions (!), I'd use the Low Normal Damage expression of the table at page 42. That is: 1d6+3 damage at 1st, 2nd and 3rd level, up to 2d8+10 at 28th, 29th and 30th level.
Mixed damage dice in the same expression is very rare in 4th edition, so I'd rule that the entire damage is fire damage.
About the attack roll... To me, it seems more a matter of accuracy than willpower and awareness. Accuracy is usually under the Dexterity or Intelligence portfolio. An attack against Reflex is appropriate, in my opinion.
To sum up, it could be something like this:
Drop a Fire Log - At-Will Terrain
The flaming log you are holding is destined to fall on your enemy's head.
Requirement: You must be holding a log on fire weighing non more than 20 lbs. and must be on a square adjacent to any square that is at least 2 squares above any square of the target's space.
Attack: Dexterity vs. Reflex or Intelligence vs. Reflex
Hit: Low Normal fire damage.