You can activate fire arrows with the
button. Your bow will turn red. Fire with
. Kill 25 enemies with the fire arrows without dying. The fire arrow takes about 5-10 seconds to respawn. This has to be done in one life so dont die. Stay back and let your allies do the fighting. I found it easy to pick enemy archers because they stand still.
In Helm's Deep, just after you defend the gate, instead of heading to the next objective, position yourself behind the right wall (which is next to the area you need to defend earlier), and then wait for enemies to appear. Kill one with a fire arrow (it's a one-hit kill), and then the rest with normal arrows. The enemies will keep respawning in small amounts, not enough to pose you any danger, while giving you all the time to charge up the fire arrows. You can hasten the process by do a
to zoom in, and then going for fire arrows headshots. From time to time, a Warrior enters here. Just kill him normally and repeat the process on the other enemies. Estimated time is about 10-15 minutes to unlock both achievements.