Re: [4E] Grandpre Academy (OOC)
Do be clear, since you had the -2 bonus on the previous attack/ambush... the net difference will be +5 (but you are right, it is +3 compared the the regular set of defenses). To give you an idea, you know that 17 VS Ac was enough... so the Ac of next round wont be higher than 22... (I'll use the same set since this still is an artillery group similar to the other).
Shadow : since you probably wont be able to get it the whole battle (once enganged, you would not be able to really use that bonus). Still, I'll grant you a +1 bonus for this power ability.