The best possible malzahar combo is null zone then holding them inside the circle with nether grasp, however if your quick enough, there is a way to increase damage substantianally.

Step 1: activate a call of the void just in front of the target
Step 2: place a null zone just under the call of the void
Step 3: activate nether grasp when the target is in the line of call of the void
Step 4: When its all over if they still arent dead quickly place a malefic visions on them and if they are low enough on health they will get killed by the visions

Note: this needs to be done quickly and some people will never be able to do this combo, it requires very quick reflexes, and judgement of where the target will be, not where they are. However if you can pull it off it reaps a great kill, especially if you want to put it on youtube.

Anyway enjoy this guide.