The Battlelord is a title of two bosses in Duke Nukem Forever.
A Battlelord is a high-ranking alien commander. It is a colossal, bipedal lizard with a powerful laser minigun and a built-in mortar launcher. There are two Battlelords in the game, one in Vegas, and the other at the Hoover Dam. The Vegas Battlelord is leading an attack on Duke's Lady Killer Casino, while the Hoover Dam Battlelord, which is much stronger, is protecting the alien "wormhole" portal from EDF assault.
Las Vegas Battlelord
Only explosive weapons and turrets are effective, so you should pick up one of the RPGs which are lying around, if you haven't done so already. The Battlelord is relatively slow, so it should be easy enough for you to fire off a barrage of well-aimed headshots with the RPG, before running around the central mound to take cover and replenish your rockets from one of the two EDF ammo dumps in the area. After defeating the Battlelord with your RPG, Duke must climb onto his head and remove a horn, thrusting it into his eye as you climb down, where Duke will finish him by repeatedly punching him "right in the jewels". He is slow, making it easy to land shots on him.
Hoover Dam Battlelord
The second Battlelord encountered is a lot harder, because it has nearly 1.5 times as much health as the first one. Duke must rip off two horns this time (and stick them into the Battlelord's head and eye) in order to defeat it, making the whole fight a two step process. There's also much less cover during this fight since Duke is on a bridge with wrecked vehicles as the only means of cover. The Battlelord attacks with his minigun, but also could harm Duke by kicking the wrecked cars. It is also aided by respawning flying Assault Troopers. When the second horn ends up in the Battlelord's eye, Duke repeats the humiliating process of punching "right in the jewels".