Timespinner - Achievement "Battle Savant" against the first boss, no New Game+, Normal

For the achievement "Battle Savant" you need to kill one boss WITHOUT using the timestop powers and WITHOUT taking damage. Usually you can do it easily on New Game+ against the same boss that I'm facing (the Feline Sentry), but you can do it easily even on your first meeting against him - I will show you how. The strategy is simple: stay at the corners of the room, near the closed doors (or you can be hitted by one of the falling rocks), stay down (to avoid the beam laser attack) and shoot your charged spell against it. When it begins to came close, attack with your melee orbs but pay attention to his claw attack: remember that he will do one single slash at the beginning, two when its life points are low. When it jumps up stay at the center of the room, near the visible rock, and when it falls down move to the opposite corner and start using again the charged spell. Repeat until it dies.