Offensive tactics first involve knowing how likely you are to win a fight. Some groups are better than others for a variety of reasons (coordination, balance, composition, leadership, experience, min/maxed builds, etc) and knowing that you are outclassed can save a lot of frustration and change the tactics that you’re should use in order to try and win an engagement.

Every player should be min/maxed for their role and responsibilities. For offinsive players that means that they should be able to do the most damage without being 1-shot or dying in in the first few seconds of a fight.

Every major patch the min/max for each class/spec may change, so always pay attention to the theory crafters (if you don’t want to do it yourself). For stamina builds, you will probably stack the most weapon damage and maximum stamina with sufficient crit. For magicka builds, it will be the most spell damage and maximum magicka with sufficient crit. You’ll need a reasonable amount of resource regeneration, and to choose the right skill on your bars. I’m not going to go into each build, though if you are in a good PvP guild I’m sure that there are people who can help you. Some high end guilds even have required builds for certain roles and responsibilities. If you’re new to the game and want a quick recommendation, I’m usually happy to spend a few minutes (as long as I’m not in battle) helping people out – regardless of your faction.

• A good balance of outgoing damage types is critical to every group.
• Coordinating your damage so that it all hits at the same time makes it much more difficult for opposing healers to keep their players up.
• The best timing on execute skills to kill opponents is based on their health. Most become effective at 20 or 25%.
• Debuffs that reduce healing mean you’ll have less damage to do in order to kill an opponent that is receiving heals from a healer (or themselves using one of their own skills). When playing in groups, have someone in a defile build. Fasala’s Guile for minor defile with a warden’s Corrupting Pollen for major defile is a good combination. This will help deal with their healing.
• Get siege on your opponents. The more damage you can as often as you can. If there are multiple players sieging, try to target the same opponents. If you see two groups about to engage, try to hit the location of the most opposing players as they engage (as so that their healers can’t keep up with the damage that your siege does and the damage that the opposing group is doing).
• People with a high enough PvP rank should be using the skills available from it, most notably detonation. This puts a bomb on you that detonates after a few seconds and damages all enemies that are close enough. There are groups who all run that skill (even those who are stamina builds). Consider that a stamina build may only do 1,500 damage with that skill (after mitigation) and a magicka build can crit at  10k. If you have 12 people all running that skill, even if they each only do 1,500 damage, their opponent is just about dead and can be killed with one tick of an ultimate.
• Knowing when to use your skills is critical. When you see a group of opponents running at you with blue glowing lights and big red circles around them (they’ve all cast Proxy Det as described above), run away for a few seconds until they detonate, and then turn around to do your damage. If you take the damage from all those detonations, then you’re reducing the likelihood of winning the engagement. Proxy Det will take a few seconds to re-arm, so you’ll have time to kill them before it explodes again.
• Tab target a player with higher importance (such as healers or the group leader – if known). Tab targeting makes things easier if you try to single target a player in order to disrupt the group. Some groups fall apart when the leader dies, others will obsess about getting their healers back up (if they run low on burst healers), and others won’t care and just continue to play and recover as normal or run to drop a camp for their dead.
• Plan to snare, immobilise, and hard cc (crowd control) your opponents. An opponent who is feared for a few seconds is not doing damage or healing. An opponent who is immobilised or knocked back is not moving with their group. There are a lot of ways to separate opponents from their groups. Wardens can use their circle to teleport them. DKs can grab one with chains. NBs can drop a fear trap in their path, and cause a few to be separated if they don’t break free quick enough. Those that become separated need to be targeted and taken down quickly.
• Be quick and accurate with Negates. Players can’t cast in a Negate. Once they’re in, they won’t be able to anything other than run towards you and hope that whatever they case previously (such as their destro ultimate) does damage while getting out of the negate – or retreat. It’s up to the leader to call for a defensive or offensive Negate. You’ll hopefully have a few of them in group.
• Prepare and practice baiting. If a few players are able to get close enough that your opponents believe that they have a good chance to get kills, they’ll likely charge and pop their ultimates. The baiting players should have Retreating, and immovable pot ready, and someone to heal them from range. As soon as the opponent ultimates are out and will only last long enough for the rest of your group to get into them, it’s time for your counter bomb.
• When running in a group, your ultimates (at least some of them) have to go off at the same time. To do enough damage to wipe another group, consider how long your opponents will be in range of your damage, how much healing they’re likely to have, and how mobile they will be. You also have to consider how much damage they’re going to do to your own group, as you have to be alive in order to do damage! This is why many groups run very tanky builds that do low damage, and rely on numbers with very few glass cannons to output enough damage to kill their opponents. A good base for medium to large groups is to have at least three destros and one sleet ready to go as soon as you hit opponents, and to try and have the engagement occur in your own negate. Remember that Earthgore procs will remove negates, so you should have a second one available at all times.
• If you don’t kill all of your opponents in the first engagement, try not to let them resurrect. You’ll probably take out half, maybe 3/4 of a tanky group if you execute your bomb or counter well. There will probably be at least one templar in Kagrenac’s Hope (faster resurrecting speed) trying to get them back up. If it looks like they won’t be able to recover, some will likely stay to delay you, while a few of them will run away to get a camp up. It’s your (leader’s) choice on how you deal with this – be it to kill the ones left, chase the runners, or let them go so that they can form up again and you can have another good fight!
• If your opponents spread out to avoid a bomb (as they should), try to go after the squishiest players. Ignore the tank with 50k health and kill the healers and bombers first!
• Against a good guild group, your approach needs to much more cautious. If they are “retreating” against anything less than 2-3 times their number, they aren’t retreating, they are baiting you into a choke to nuke you. Do not follow into the chose. Let other players make them use all of their ultimates or bait them out if you can, and then hit them when they are unable to fight back.