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This is just some simple advice in response to many other threads. It took me until about level 50 to really see how important combos can be, especially after some not so pleasant explorable mode dungeon experiences, but I've come to realize how well coordinated combos can be game changers. I suspect that in the not so distant future, players will primarily base their weapon choices in groups on what types of fields they lay down or what types of finishers they provide more than on the value of the individual skills themselves.
The following is a small list of some combos that I've found to be extremely useful in group situations. For a full list of available combos and their effects, visit the wiki page.
Need to heal more effectively? Use combos!
Sure, everyone has their own personal heal and is largely responsible for his or her own well being, but there is plenty you can do to support each other in the healing department. Most effective of all is good use of combos through water fields. If your group is in rough shape, gather, call out for a water field, then have mutiple people use a blast finisher inside it. It's guaranteed to get your whole group back up to full!
Even if your group is in alright shape, water fields in combination with leaps, projectiles and whirls can be a good way to keep players regenerated and topped off. If you're dropping the the water field, be sure to place it in the path of ranged DPS like rangers so you land the most projectile combos (this applies to most fields.) In fact, it's often smarter to place a field in between players and enemies than on top of enemies themselves! It's a decision you'll have to make on the fly.
Need more effective CC? Use combos!
Many individual CC skills do not last very long in GW2. Effects like chill, however, stack in duration. A well comboed ice field with projectiles can keep hard hitting melee mobs slowed for upwards of 20 seconds!
Blind rotations are insanely effective against most physical mobs. But some groups may be limited in the number of blinds they individually have available to them, or you may be faced with a pull of multiple deadly melee mobs while armed with only several single target blinds. What is a group of adventurers to do? Enter combos! Blast finishers through a dark field produces a very effective aoe blind. Every single Necromancer staff skill creates a dark field. Properly coordinated blasts with each necro mark can permanently blind every enemy in the area!
In over your head? Use combos!
Perhaps my favorite combo invovles the use of a smoke field and a blast finisher. This gives all allies in the area aoe stealth! This can help you escape some sticky situations. Used in conjunction with a water field and blasts to get your party back to full health, this can be a real game changer.
Feeling weak? Need to hit harder? Use combos!
This is perhaps the most basic combo set of all, but I don't see a lot of groups consciously utilizing it yet, so it bears mentioning. On each pull, your goal should be to try to weaken the enemy as much as possible and strengthen your group as much as possible.
A well placed lightning field (only available with a staff elementalist) and a bunch of projectiles, combined with individual abilities that apply vulnerability can make your enemy take up to 25% more damage. Anyone who has number crunched in MMOs before knows just how significant this can be. On super bosses, you'll want to reapply vulnerability as much as possible.
To coincide with weakening your enemy's defenses, you should boost your allies' offense. A good fire field with several blast finishers can give your entire group a huge power and condition damage boost. At level 50, a well comboed might stack (around 20 stacks) gives my entire group a whopping extra 500 power and condition damage!
I notice this combo happens fairly frequently naturally, but only occasionally, and in much smaller amounts. Coordinating both of these combos before and during each pull can significantly boost your party's power. The same applies to pvp.
There you go, these are just a few very useful combos that me and my friends have messed around with so far. There are many many more good ones that will require a lot of practice and teamwork and can be equally useful. Until a full combo guide gets released, have fun experimenting!
To everyone else, feel free to post your favorite/most useful combos so far.
Editted formatting. I wrote this during my last hour during work in between taking care of customers