I managed to get him playing a soldier (level 55) on insanity. I was using a Black Widow and Avenger, both level 5; my armor, intel and skill bonuses were maxed for headshot damage then weapon bonuses. My second squadmate was Garrus (Phaetus & Black Widow).
The first thing I did in the battle was pop and adrenaline rush and use the time to get away from him without engaging in melee. This allowed to get some of his shields down quickly so that he destroyed the first segment of floor while regenerating. This was extremely helpful - if you engage him too long without cover his warp bullets will overwhelm you and force you to use many medkits in a row before your shields recharge.
I don't know about the lower difficulties, but on insanity the troops he summons pose the largest threat: Assault troopers flushed my position with grenades so that the snipers/phantoms could finish me off. Eliminate them first and Leng will be easy picking. For this, I used the Avenger loaded with inferno rounds to down the troops and had EDI keep a decoy up to draw fire and grenades.
It helps to keep your squad separated. By having them hold different positions you thin out the amount of grenades you experience in your hole, and allow for crossfire to pick off the enemies faster. If Leng attempts to melee one of your squadmates, it is a free headshot too.
Through the whole battle I managed to keep Leng from recharging his shields with a BW round to the head every so often. Here adrenaline rush is crucial - a miss wouldn't do, plus the damage bonus helps significantly.
Once his shields are down he is cake: a single headshot with a disruptor round took his head clean off. The battle may not end on his death if any troops are remaining.