Thought that this would be a good adjunct to Nightsong's awesome OP.
Below - originally came from http://www.shadowrefuge.com/
The original also has some nice video's and wiki links to various GW2 items explained below.
Cross Profession Combos - A Closer Look
After BWE 1 there were a lot of threads opening up for discussion on the various professions and the viable builds and strategies they could employ to be successful in PvP. One aspect of the game that I found was often overlooked was combo fields and finishers. As it stands now there are 9 types of combo fields and 4 types of combo finishers. Each field offers varying effects based on the type of field and finisher, and I believe these fields will be the key to controlling the battlefield and gaining momentum in team based PvP. Being able to apply tons of nasty conditions to enemies, valuable boons to allies, or healing across the battlefield can prove to be a key component in your success in both structured PvP or World vs World PvP.
Let's do a brief rundown of the 9 types of combo fields and what they generally can bring to a battlefield.
Dark - Adds Blindness to attack for both blast & leap finishers. Applies random conditions on attack for projectile finishers.
Ethereal - Applies Chaos Armor to allies for both blast & leap finishers. Applies Confusion to enemies on attack for projectile finishers.
Fire - Grants Might to allies on blast finishers, applies Fire Shield to allies on leap finishers, adds Burning to projectile attacks, and sends fire flying on whirling attacks
Ice - Applies Frosty Armor to allies for both blast & leap finishers. Applies Chilled to projectile attacks.
Light - Grants Retaliation to allies for both blast & leap finishers. Removes conditions from allies near attack for both projectile & whirl finishers.
Lightning - Applies Weakness to attack for blast finishers. Adds a lightning based buff to allies for leap finishers, and applies Vulnerability on projectile attack.
Poison - Adds weakness to attack for blast finishers, and Poison to projectile attacks
Smoke - Cloaks nearby allies on blast finishers, cloaks yourself on leap finishers, and adds blindness to projectile attacks
Water - Heals nearby allies on blast & whirl finishers, heals allies near attack on leap finishers, and heals allies attacking the same target for projectile attacks.
Now that we have that out of the way I want to take a closer look at combo fields and finishers, which professions can access them and how easy are they to access. I'll go through and break down which professions have access to which combo fields as well as which professions can use which types of finishers. We'll start with combo fields:
Necromancers have access to (4) Dark Fields, (1) Ethereal Field with Spectral Wall, (1) Light Field with Well of Blood, and (2) Poison Fields with Poison Cloud and Chillblains.
Thieves have access to (1) Dark Field with Shadow Refuge(!), (1) Poison Field with Choking Gas, and (2) Smoke Fields with Black Powder & Smoke Screen
Elementalists have access to (5) Fire Fields, (1) Ice field with Frozen Ground, (1) Lightning Field with Static Field, and (2) Water Fields with Geyser and Healing Rain.
Mesmers currently have (4) Ethereal Fields, and (2) Light Fields with Temporal Curtain and Veil of Invisibility
Engineers can use (2) Fire Fields with Fire Bomb and Napalm, (1) Light Field with Super Elixir, and (1) Smoke Field with Smoke Bomb.
Warriors have access to (1) Fire Field with Combustive Shot.
Rangers currently have (2) Fire Fields with Bonfire and Flame Trap, (1) Ice Field with Frost Trap, (3) Poison Fields from the Spider, Murrellow, and Carrion Devourer pets, (1) Smoke Field from the Jellyfish pet, and (1) Water Field from Healing Spring
Guardians can use (9) Light Fields and (1) Fire Field with the ability Purging Flames.
Warriors only have 1 combo field which comes from their burst skill on the longbow Combustive Shot.
Guardians only have 2 types of combo fields but they have the most combo field abilities with (10)
Of the 9 possible combo fields for the Elementalist 6 of the abilities are on the staff.
Static Field the staff ability for the Elementalist is currently the only Lightning Field in the game
Only two professions can create Dark Fields, Necromancers & Thieves
Only two professions can create Ethereal Fields, Mesmers & Necromancers
Rangers & Elementalists are the only professions that can create Water Fields or Ice Fields.
Rangers have access to the largest variety of fields with (5).
Now we'll move on to finishers, there are 4 types of finishers Blast, Leap, Projectile and Whirl. Not all finishers effect all fields, currently blast & projectile are the only finishers that have an effect for all field types. Leap has an effect on every type except for poisons, and whirl finishers only effect water, light, and fire fields. I'm going to break down how many finishers of each type each profession possess.
Necromancers have (4) Blast Finishers, (1) Projectile Finisher with Necrotic Grasp, and (1) Whirl Finisher with Deadly Catch
Thieves have (1) Blast Finisher with Cluster Bomb, (1) Leap Finisher with Heartseeker, (14) Projectile Finishers, and (2) Whirl Finishers with Leaping Death Blossom and Dagger Storm.
Elementalists have (9) Blast Finishers, (5) Projectile Finishers, and (1) Whirl Finisher with Dust Devil.
Mesmers have (4) Leap Finishers and (2) Projectile Finishers with Mirror Blade and Trick Shot.
Engineers have (7) Blast Finishers, (1) Leap Finisher with Jump Shot, and (4) Projectile Finishers.
Warriors have ( 8 ) Blast Finishers, (4) Leap Finishers, (10) Projectile Finishers, and (4) Whirl Finishers
Rangers have (1) Blast Finisher from Tail Swipe an ability on the Drake pet, ( 8 ) Leap Finishers, (12) Projectile Finishers, and (1) Whirl Finisher from Whirling Defense.
Guardians have (2) Blast Finishers from Mighty Blow and Shield of Wrath, (1) Leap Finisher with Leap of Faith, (3) Whirl Finishers with Binding Blade, Protector's Strike, and Whirling Wrath.
Thieves, Warriors, and Rangers are the only 3 professions who have access to all 4 types of finishers
Thieves using 2 Daggers will have access to 3 finisher types on 1 weaponset
Thieves using Shortbow & Dagger/Dagger will have access to all 4 types of finishers, and are the only profession who can achieve this without the use of utility skills or pets.
Thieves have the most projectile finishers with (14), Rangers are a close 2nd with (12), followed by Warriors with (10)
Elementalists have the most blast finishers with (9).
Rangers have the most Leap Finishers with ( 8 ) however of the 8 abilities (6) of them are pet abilities.
Elementalists have (5) projectile finishers (2) of which are on the Staff in Earth Attunment, (2) are on the Scepter in Earth Attunement, and one is the Utility Skill Arcane Blast.
Warriors have the most finishing abilities at (26)
Guardians, Necromancers, and Mesmers all have the least at (6)
I think that when planning a build or a strategy for a team combo fields need to be considered. There are several instances where a strategy could even revolve around the use of certain fields, for example Static Field the Elementalist staff ability can be used to lock down a target with a stun and focus them down, with vulnerability added to them from every projectile attack on your team. If you're facing a team with lots of condition removal the Dark Fields of a Necromancer can be greatly useful in rapidly stacking conditions and reapplying them to the enemy team. Proper use of a Water Field could greatly add to your team's overall survivability in a fight, and give you a lot of momentum. There are so many possibilities to consider when constructing your team makeup as well as your builds, or even World vs World tactics when fighting in large groups.
All of these abilities and effects may likely change several times before launch, but I want to emphasize how important they can be and encourage everyone to think about them more.