Recently, I was trying to follow my own advice and run a very difficult combat encounter that involved varied terrain, non-standard situations, and a decent number of enemies and enemy types.
I found myself repeatedly forgetting that certain enemies had certain abilities, and I kept making poor tactical decisions. This was particularly bad in the case of the wizard, which was high level and had an extensive spell list. These slip-ups ultimately made the encounter much easier than I intended.
How do I handle this issue when I DM? Other than the glib "don't forget anything," are there any table-tested techniques or tricks that keep you from forgetting or overlooking abilities?
I'm running my games partially online (some players physically present, others video in), using Roll20, if anyone has suggestions that are specific to that platform.
Prepare your tools
I find it's helpful to prepare your tools. It is a time investment to do this, but the idea is that you move the hard work to the DM prep time so you don't have to do it during the actual game.
Flash cards: one card per monster
This could just be index cards. I hand-write each one myself. The practice also helps me remember everything so that each card becomes a guide rather than a crutch.
List HP, AC, traits, immunities, etc. Order them according to Initiative.
List turn actions. Ex: Turn 1, cast Hold Person; Turn 2, cast Booming Blade.
This trades flexibility for speed. When you, as the DM, find that you are overwhelmed by the number of actions and options your monsters can take, you have too much flexibility. Reduce your cognitive load by determining the monsters' actions before the battle.
This allows you to quantify exactly how powerful a monster will be during the encounter, both in terms of damage and support.
This may not always be possible, or you might sometimes need to deviate from the plan. But your goal for big fights should be to design encounters that enable you to stick to the plan.
List reactions and triggers, with emphasis
Pre-roll attacks, damage, and ability checks
Cheat Sheets: a handwritten list of encounter-relevant info that replaces the DM screen
The primary benefit of a DM screen is it hides your rolls. Tacking general info onto it can be useful too, but the DM screen info cannot ever be tailor-fit to serve each encounter. If you're running a big encounter, the screen will not be optimized to help you run it. Instead, use only one sheet of paper that you will write various conditions, hazards, and other notes that you think will be useful for you in this specific battle. Use only this and ignore the DM screen's info.
Take a second letter-sized paper, fold it in half length-wise. Use the left side to outline the flow of battle you expect to happen, but do not make it too specific. Use the right side as "post it note" space -- details that you think will be relevant at each major point in the battle's flow. For example: the DC for the trap door, the weight capacity of the bridge, or the specific reminder that this area is dimly lit. Try to limit yourself to only one page per encounter. Don't use the back of the first cheat sheet because you want to have both sheets handy at the same time.
Handouts: information -- maps, trivia, etc -- you give to your players ahead of time. This will let them, if they take the handout seriously, to pre-ask all the questions you consider trivial during the encounter.
Your players may not always take the handout seriously. They might receive it and read it, but forget about it 10 minutes later. So only fill it with info that you think they should already know, such that if they ask "what do I know about X?" or "how do I get from A to B?" you can just say, "It's in your handouts." This reduces the work you have to do to run the encounter.
Full Disclosure: I always run Theater of the Mind, so using maps as handouts may be more useful for me than another DM who uses grids.
Practice the battle
Run the battle over the course of the week. After a certain point, you may start considering options you didn't initially think about. This includes better strategies for your monsters, abilities you didn't know you had, or fun new twists you didn't think about previously.
This is also why it's good to do practice runs over one week. If you do it in one day, you might miss out on some of these new techniques. But if you sleep on it and come back, you might have a new insight you didn't previously have in your last test run.
Enforce order at the table
This may not be a problem for you, but it's important to keep everybody focused at the table, because some players might derail or distract you or the others. This equals more work for you as the DM, and so it may lead to you forgetting more things.
Ask the players to put their phones away. It also helps if you've built up the tension leading to this encounter, so they are on the edge of their seats and don't want to look away (use music to set the mood).