Warlock tactics

Initial considerations
While as with any class there are multiple different ways of building a Warlock, given its available skillset and the Eldritch Blast a Warlock is best used as a Skirmisher; constantly firing ranged damage at enemies while keeping out of range of enemy counterattacks as much as possible (a tactic also known as Kiting). To do this well, consider the following principles:
 * Power your blast. There are many ways of increasing the strength of your Eldritch Blast attacks, and the more you can use without sacrificing defense or mobility the better.
 * Know your shapes. Focused, Cone and Chain are the three cornerstone shapes of Eldritch Blast, and each of them shines in different situations. Have all three hotkeyed and ready to switch to at a moment's notice as the numbers, positions and types of enemies you encounter changes to suit each one better as a quest progresses.
 * Be mobile. Always make sure you can move well. You must have raw speed such as through Expeditious Retreat (and the Feeding Frenzy enhancement within Soul Eater enhancements which has the rather rare ability to stack with other speed buffs), the ability to safely drop from height via Feather Fall (through spell, equipment or enhancement), and the ability to do long underwater travel (a good stock of Potions of Water Breathing handle this easily). There are also quite a number of situations where Invisibility will be highly useful, such as sneaking up to/past enemies that are likely to raise an alarm so that you can rapidly kill them or just avoid them instead. These are the absolute minimum; additional effects like an increased Jump stat (or spell) are nice but not explicitly mandatory.
 * Know your environment. You will be constantly backing off of enemies while firing at them to keep damage off of you, so you will need to be well aware of your surroundings at all times. Backing yourself off a cliff, into a corner or into another bigger squadron of enemies will not end well for you.
 * Don't ignore defense. Even with proper positioning and kiting you will still get hit frequently, so you need to be able to endure it (and a Warlock is better than average at this among spellcasters). Make sure you build up and activate your defenses against each potential enemy type:
 * Melee enemies are among the most vulnerable to you. Cone shape to drop them fast, high movement speed such as via Expeditious Retreat or Haste, semi decent AC through good armor and effects like Shield and Stoneskin, activating your various Enhancements that increase PRR and Fortification, and finally evasion effects such as from Blur and Displacement can all combine to help you handle such enemies.
 * Physical ranged enemies (like archers) you won't be able to kite; they will in fact outrange you, so always keep that in mind. However, the physical defenses used for Melee enemies such as AC and evasion effects also work on archers, and you will quite typically out-damage them thoroughly. Chain shape is typically best against archers on far-away platforms in small groups, though if you are dealing with just one of them you could also use Focused. With Chain in particular, consider options for damaging the archers outside of line-of-sight (and therefore they can't hit you either); could you bounce a Chain off of rushing melee enemies or perhaps a nearby crate to hit the Archer? Or, failing that, can you position yourself so that you are in line-of-sight of just one Archer, so you only have to endure fire from one of them while Chaining into two others as well? Evaluate the area the archers are deployed in before getting into the line of fire
 * Magical ranged Damagers are the ones that shoot damage-inflicting spells like Frost Lance, Fireball or Lightning Bolt. These can be difficult to avoid and they hit hard. However, a Warlock has a whole host of defenses to handle it; multiple natural sources of MRR, barrier spells such as Protection from Elements or the VERY effective Resist Energies from the Enlightened Spirit enhancements tree, and careful selection of Equipment can let a Warlock take these hits better than average. One additional tactic that has been found rather effective against Magical Ranged Damagers who tend not to do any physical attacks: they can't easily handle a fast up-close enemy. Have the Blast in Cone form, have the double speed boost from both something like Haste and the Feeding Frenzy enhancement on Consume active, get right up into point-blank melee range and constantly fire the Blast while side-strafe circling them. At that range your position angle relative to them changes very fast, and for a large number of linear bolt-like spells the caster cannot change their firing direction while in the middle of the casting animation. With this rapid circling, in the time it takes a caster to complete the casting animation for a bolt you will frequently have moved outside of its targeting angle and the spell will completely miss you.
 * Magical Crowd Control are the enemies that shoot the various debuff abilities and skillcheck attacks like Finger of Death. Definitely a nuisance to anyone, but a Warlock has several options for these too: natural Fear immunity and boosts to rolls against Poison and Disease, the spell Nightshield, equipment that boosts Spell Resistance and various others should be carefully selected.
 * Don't ignore healing. Despite all of the above you will still get hit, and especially on Reaper difficulty you will need ample sources of healing. Steal Life Force at its max upgrade can match the per-shot effect of multiple casts of Cure Critical Wounds (such as from a Wand you can also use with properly trained UMD). And make sure to carry a good spread of potions/scrolls for curing Curses, Blindness, Negative Levels and the like.

Special considerations
There are a few more specific situations that a Warlock will need to consider their tactics and build carefully.


 * Traps
 * By itself, a Warlock can't disable these. At higher difficulties like Reaper mode, the cushion of temporary hit points often is not enough to survive a trap strike, so you will need to be able to disable them.
 * If you can't have regular access to a Rogue ally, you'll need to do it yourself. Multiclassing one single level into Rogue or Artificer gets you access to the skills needed to find and disarm the trap Control Panels (as well as the ability to pick locks). If you intend to do this, it's best to take the trapper class at the very first level. This does have a cost. Remember that Warlocks receive only two skill points per level, and none of the needed skills are class skills for them - you will need to invest in Intelligence. You lose one 1d6 die of core Blast damage, one 1d4 die of Pact Blast damage and the ability to access the very final Core enhancement in your three trees, as well as a couple of other minor enhancements. But traps in high-difficulty missions can be so dangerous that some players prefer to disarm them themselves.


 * Restrained missions
 * Blast-focused Warlock players typically like being walking death cannons. Restrained missions, i.e., missions where you are NOT supposed to kill or break everything in sight and will fail the mission if you do so (such as The Faithful Departed or Stealthy Repossession) can end up being very, very difficult and frustrating for a Warlock since it can be frighteningly easy for them to kill whatever they're not supposed to before they even know they're hitting it. Study these missions CAREFULLY before you enter them, restrict yourself to the explicitly single-target Focused shape whenever necessary, and even make sure you manually lock on to other enemies (default right-click on them with a mouse) to prevent accidentally auto-targeting the enemy or breakable you aren't supposed to destroy.
 * This can be even more difficult for aura warlocks, where enemies get hurt just by stepping into your aura.


 * Heavy Turret enemies
 * Typical blast-focused Warlocks have very good sustained damage over time, but this also requires them to stay within line-of-site of enemies and therefore able to be targeted by ranged attacks. A number of ranged enemies tend to park themselves in place like a turret and fire off attacks when you come within view, and for some enemies like an Elder Beholder it can be quite dangerous to stay in view of the enemy for long stretches of time. For these kinds of enemies, and if the surrounding terrain can give you ample cover for it, a hit-and-fade tactic is preferable; you manually lock-on to the target (default right click with the mouse on them), prep a ranged attack (for the Blast, likely in the long range Focused stance), pop out from behind cover for just a brief instant and fire it then immediately move back behind cover hopefully before the enemy can hit you with a counterattack, rinse and repeat until they fall. For the Eldritch Blast and most spells, there is a very slight delay between the time you press the attack button and the time the attack actually fires. Hit-and-fade works quite well if you have learned this delay so you can activate the attack while you are still behind cover (so you will just hit the cover if you don't move), pop out at the very last instant just before it fires so you get a clear shot, then immediately move back the instant it fires so that you are exposed for the absolute minimum amount of time. For the particular example given of Beholders this has proven quite effective.