My two cents: Use Firework runes. The 25% movement is needed for core guard, and the start of combat boons help the only small niche core guard's still have in WvW; teleporting on unsuspecting people and bursting before they can react. That being said, I would highly suggest you switch Zeal for Virtues if you want to remain core guard. The ~20% damage increase doesn't outweigh the utility Virtues offers, and will help you with the survivability you are lacking. As for stacking boon duration ... don't. You shouldn't be striving for boon duration to just have longer retaliation, you should be making sure you have on demand retaliation (IE symbol of wrath, traited Virtue of Justice, Litany of Wrath, etc) prior to using big burst damage abilities. Finally, if you want to preserve your playstyle, but gain more advantage consider speccing into Dragonhunter. It can be played like core guard, but gives access to stronger Virtues in larger scale conflicts as well as access to traps. The longbow can be use to help when fighting zergs because even though it isn't great it still provides better range pressure than other guardian range weapons.
