This is somewhat of a meat and bones topic for a primer. It involves a large amount of threat assessment, table feel and knowledge of your deck and what possible decks your opponents will have. As you gain more information regarding what you are playing against you should be able to piece together a general idea of what strategy your opponents are pushing. Are they a combo deck? What sort of combo?
Aetherflux Reservoir
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Tooth and Nail
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Deadeye Navigator
? All of these decks interact with other decks differently and you have to be aware of how they execute their game plan. Choosing what to interact with and when to pull the trigger on a piece of removal or counter magic is crucially important to learning how to play a control deck. One thing that sets this style of play apart from Stax style control decks is that we do not proactively deny resources or prevent people from casting spells on a regular basis. We let the vast majority of spells go through or leave them to be dealt with by our other opponents. We pick and choose exactly what we interact with so as to keep everyone at bay. It is a tenuous balance and takes consistent reevaluation at almost every turn. To assist with this I personally work through a series of questions while analyzing how I want to interact with a spell that is being cast.
Do I care about it?
This is the first question to ask. They have their own gameplan in mind, whether it is trying to win or assemble an impenetrable board state everything they do will be pushing towards that game plan. The first thing to consider is how does this affect us and our board state. Is it a mana rock? That really doesn’t affect us or our resources, we don’t care about it from a territorial perspective. If it does affect our resources we need to carefully weigh whether the loss of resources is worth the expenditure. Sometimes our loss is far less than another’s and we are willing to take the hit in order to take another player down a rung. In other situations it is our board that is being hit the hardest and we need to protect our lead. There is a careful balance, the big thing of note is that we have to build our advantage slowly. Other decks are able to deploy their wins at the drop of a hat and we need to be very cautious about spending too many resources protecting things that in the end will not win us the game.
Is this a Threat? Does this affect my victory or my resources?
The big thing to think of here is, does this actively prevent us from winning the game or executing our game plan. We need to evaluate our game plan and how this threat is going to effect us. We want to allow our opponents to deploy threats and have other players be scared by them, but we need to be sure that those threats aren't insurmountable.
Does this threat need to be dealt with?
When we have identified that a spell or permanent is a threat we then need to identify if it needs to be dealt with or lived with. Several threats we can ignore as they do not affect us as much as other players at the table. We need to identify who all is affected by the current threat. Is it just us? Are others more or less affected by the current threat? If we feel that it is hurting others more than us, can we live with it? If we have determined that something needs to be dealt with, next we need to figure out who needs to deal with it and when.
Who deals with this threat?
If it has to be answered on the stack, can we interact with it? If we can interact with it, what is the priority order? Are we before or after the other blue players in the priority order? If there is no other Blue player behind us, most likely we will need to interact with this on the stack. If there are other blue players behind us, will they see this as a big enough threat to deal with it? Do they have the resources to deal with it? Have they used any interaction recently? How much mana do they have? If they haven’t interacted much and have open mana, passing priority to them may force them to deal with this threat for us. It’s free resources to us, why let it go to waste?
Do I need to Deal with this?
This is a very situational decision. Our main goal is to interact with players before they hit critical mass. To poke their one point of weakness right when it matters the most. We want to wait as long as possible to expend resources. As opponents get closer to their goals, each one of them will start moving up on the priority list. We need to weigh if other opponents see this as a big enough threat that it will stop them from completing their goals. If not, the responsibility will fall on us to take care of it.
Do I deal with it Now or Later?
Does this threat need to be answered on the Stack or on the Battlefield? If on the battlefield, do we care if it sits around for a while?
Is this the right opportunity?
This is looking at the table position. Is this spell going to win the game for a player, but if I interact with it, will just let another player win before I untap? Can we look at an alternative way to interact here? Is this player the one right before our turn? Be aware of what the table situation is and make others aware of it too. The longer the stalemate goes the better off we are. The stalemates give us more time to accumulate resources and more time for our value engines to run. If it isn’t a stab at breaking the stalemate we probably should let it be.
How do I deal with it?
We have multitude of ways to deal with things Mass Removal, Targeted Removal, Player Removal, and Creature Beats. With 8.5 Tails, mass removal is our primary interaction point, however we have to be sure we are using the correct tool. Some board wipes will clear out noncreature permanents. We need to make sure we save those for critical times. When our opponents can't quickly rebuild and when we won't unduly damage our own board state. We do have some targeted removal, which isn't to be neglected. They are a surgical weapon, use them sparingly and cautiously. Player Removal is not to be scoffed at. Choosing who to attack and sticking to a single target until they are gone is surprisingly effective. If we know that we will not be able to interact well with that player in the late game, apply pressure to them early with the creatures that we do deploy. Once we get into the later game, our creature beats become much more effective. Having players low means that the larger creatures can end them in 1-2 turns instead of 3-4 which is a huge difference in what our other opponents can muster for a defense.