Lan can fly - so he can move, attack and get you moving onto the next dragon quicker. He cannot take out a dragon on his own, however.
If the target dies before he can help, Lan will redirect attacks at a different dragon (the one casting spells).
Oh, and I screwed up some rolls.
Att 1 - (1d20+18)[26], Skirmish - (6d6)[24]
Att 1 - (1d20+13)[23], Skirmish - (6d6)[25]
Att 1 - (1d20+8)[28], Skirmish - (6d6)[16]