I'm not going to cite specific cards here because I don't have them in front of me, but it should be clear what I'm talking about.

Let's say a unit (let's call him Steve) with 2 HP and Toughness 2 is defending against a unit with 2 power. After the apply damage step, 2 damage is applied to Steve. Then the attacking player plays a tactic that deals 2 damage to Steve. What happens? Does the Toughness apply to both sources of damage, thereby rendering 0 damage done to Steve? Or does Steve have 4 damage applied, and Toughness reduces 2 but he still dies?

Good question.

From the rulebook: "Whenever a unit with the Toughness keyword is assigned damage, the Toughness keyword cancels its numeric value of that damage before that damage is applied. Cancelled damage is not applied to the unit."

Since damage from Tactics cards is assigned separate from the Battlefield Phase - Assign Damage Step, it would seem that Toughness does negate 2 from the Tactics card. Tactics damage is assigned and applied immediately, unlike Battlefield Phase damage.

A caveat to this is that if you assign and apply damage via a Tactics card during the Action Window after Attackers have been declared, the Attacker still deals his damage, but is sent to discard after the Tactics resolves. If you, the Attacker, direct damage/kill a Defending unit with a Tactics card after Defenders have been declared, the Defender still deals damage, but is discarded after your Tactics card successfully resolves, thus allowing your attackers to not have to include the Defender as part of their damage distribution.

Correct me if I am wrong somebody (=

I believe the only way to do damage during combat from a no-combat source and still have the unit (attacking or defending) do its damage is if the tactic resolves after the assign damage window but before the apply damage step. All damage is counted and pooled during the assign damage window and no actions can be taken until after this step.