Fighting average targets:
In most cases, summoning a Felguard to fight with you will always be the best solution. Prioritize the Felguard's attack and then cast affliction spells in sequence of corruption firstly, and then curse of agony. Felguard will have already been fighting the target when a warlock needs to spend 3 sec to cast them. Felguard is able to draw and control all Aggro in this process while on the other hand his master just needs to send shadow bolts to kill. Remember that it is always the best time to drain souls and get a soul shard when target's health decreases to less than 30% of its maximum.
A warlock will be often short of mana or health in a series of battles. At this point, send the Felguard to fight and meanwhile use Life tap to convert health into mana, and then drain the target's life to supply health. Certainly it takes longer than directly using shadowbolt to kill, but occasionally using this strategy makes a warlock to fight nonstop.
Fighting powerful targets:
Voidwalker is preferable while fighting powerful targets. His high level defense, ability to avoid and reduce damage combined with his durable health makes it possible to always fight behind and let him bear all the damage. What you need to do is to cast afflictions and shadowbolts spells. Although a voidwalker can't attack targets as severely as a felguard, his Torment will draw all targets' Aggro and control the crowd. Please note that if your target is over-powerful or possesses high volume of health, it will take longer to kill and Health funnel has to be used to recover the Voidwalker's health now and then.
A stable cycle exists in this fighting strategy: Life tap converts health into mana, health funnel consume mana to give health to the voidwalker so that he resists more damage. Life drain turns your target's life into yours while affliction spells has effect on and reduce the health of the target. The disadvantage of this method is that the low average damage will result in the longer process of the battle. It is thus regarded as an effective secure method when trying to accomplish multi-player elite quests or facing elite mobs.