You have to do the riolu kid like usual, and The big monster. These are the correct lines:
1.Over here! M.B.!!!
2.I'll turn you to scrap!!!
This is all the lines.
Moves to use on certain scenes:
1: Slowbro's light screen
2:Slowbro's Sunny Day
(Slowbro dies here, use any move)
3:Typhlosion's Overheat
4:Thphlosion's Overheat
This should do it.
Source: http://serebii.net/black2white2/pokestar/thegiantmonster.shtml