9. Krauser - Resident Evil 4
No matter how tough a boss is, you should be able to defeat them if you just learn their attack pattern. However, fighting Krauser in Resident Evil 4 is easier said than done. The psychotic soldier keeps adjusting his strategy and changing the battleground so you never get too accustomed to the area.
As you search for the three insignias to unlock a door, Krauser blasts you with a machine gun, hurls flash grenades and explosives at you, and sends mobile mines and drones to cut you down to pieces. He tends to attack you from a distance but occasionally tracks you down, forcing you into hand-to-hand combat.
Once you obtain the second insignia, Krauser mutates his arm into a blade and sets C4 explosives to blown the place to kingdom come in three minutes. This is, by far, the hardest part of the fight since Krauser shields his body with this bulletproof blade, meaning you can't hit him until he tries to hit you.
Because you're against the clock and Krauser has an instant-kill attack, you can't rely on subtlety or patience. You need to unleash everything you have to put him down for good as quickly as possible.