crude approximation, but it works.

OpenRa.Game/Effects/Demolition.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using OpenRa.Game.Traits;
+
+namespace OpenRa.Game.Effects
+{
+	class Demolition : IEffect
+	{
+		Actor attacker;
+		Actor target;
+		int delay;
+
+		public Demolition(Actor attacker, Actor target, int delay)
+		{
+			this.attacker = attacker;
+			this.target = target;
+			this.delay = delay;
+		}
+
+		public void Tick()
+		{
+			if (--delay <= 0)
+				Game.world.AddFrameEndTask(w =>
+				{
+					w.Remove(this);
+					target.InflictDamage(attacker, target.Health, Rules.WarheadInfo["DemolishWarhead"]);
+				});
+		}
+
+		public IEnumerable<Renderable> Render() { yield break; }
+	}
+}

OpenRa.Game/OpenRa.Game.csproj

@@ -81,6 +81,7 @@
     <Compile Include="Chrome.cs" />
     <Compile Include="Combat.cs" />
     <Compile Include="Effects\Corpse.cs" />
+    <Compile Include="Effects\Demolition.cs" />
     <Compile Include="Effects\MoveFlash.cs" />
     <Compile Include="Effects\Smoke.cs" />
     <Compile Include="Effects\TeslaZap.cs" />

OpenRa.Game/Traits/Activities/Demolish.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using OpenRa.Game.Effects;
 
 namespace OpenRa.Game.Traits.Activities
 {
@@ -18,11 +19,8 @@ namespace OpenRa.Game.Traits.Activities
 		public IActivity Tick(Actor self)
 		{
 			if (target == null || target.IsDead) return NextActivity;
-
-			// 1. run to adj tile
-			// 2. spawn timed demolition (for +3/4s)
-			// 3. run away --- where?
-			return this;
+			Game.world.AddFrameEndTask(w => w.Add(new Demolition(self, target, 25 * 2)));
+			return NextActivity;
 		}
 
 		public void Cancel(Actor self) { target = null; NextActivity = null; }

OpenRa.Game/Traits/C4Demolition.cs

@@ -24,7 +24,9 @@ namespace OpenRa.Game.Traits
 			if (order.OrderString == "C4")
 			{
 				self.CancelActivity();
+				self.QueueActivity(new Move(order.TargetActor, 2));
 				self.QueueActivity(new Demolish(order.TargetActor));
+				self.QueueActivity(new Move(self.Location, 0));
 			}
 		}
 	}