To unlock Expert difficulty, beat the game on Hard difficulty. To unlock Ultra difficulty, beat the game on Expert difficulty.

Unlock the following weapon upgrades by performing the corresponding tasks:

WEAPON UPGRADE - HOW TO UNLOCK
Assault Rifle level 2 - Get from the box in Mission 5.
Assault Rifle level 3 - Beat the game on Ultra difficulty.
Grenades level 2 - Beat Missions 1, 2, and 3 on Normal difficulty with an "S" rank.
Grenades level 3 - Beat Missions 1 to 8 on Hard difficulty with an "S" rank.
Rocket Launcher level 2 - Beat Missions 9, 10, and 11 on Normal difficulty with an "S" rank.
Rocket Launcher level 3 - Beat Missions 1-12 on Ultra difficulty with an "S" rank.
Shotgun level 2 - Get from box in Mission 11.
Shotgun level 3 - Beat the game on Expert difficulty.
SMG level 2 - Get from the box in Mission 3.
SMG level 3 - Beat the game on Hard difficulty.
Whip level 2 - Beat Missions 5, 6, and 7 on the Normal difficulty with an "S" rank.
Whip level 3 - Beat Missions from 1-8 on Expert difficulty with an "S" rank.

To target multiple enemies with the rocket launcher, hold B.

Destroy the hives as soon as possible as enemies will keep spawning from them.

Keep some distance away from the hives as enemies will spawn faster the nearer you are to them.

Let the enemies come to you. After taking out a wave, go around and take out the enemies blocking the radar. You can also use the tank like a turret if needed.

-Set a tank to self destruct by pressing A. The explosion may take out a group of enemies.

-Hold B when firing the rocket launcher to target multiple enemies and fire multiple rockets.

-Use the Laser Whip to save ammo. It recharges and will not run out of ammunition. You can use it to take out up to three enemies at a time.

-Even though the shotgun is less useful at a distance, you can take out a group of enemies by rushing them and firing a couple shots.