Here is a list of random but useful tips that should help you out to get better in combat:

One character - One strategy it's best that each of your character does what it is supposed to instead of trying to do everything with every character.

Be ready, be adaptive It's good to specialize your characters but don't rely too much on the same strategy or you may have serious troubles in some situations. You will face very varied encounters and if for example your main source of damage is fire any Magical Armor heavy or Fire resist heavy team will completely destroy you. It's also good for each of your character to do a tiny bit of what it's not supposed to do such as having one Magical Armor CC on your heavy Physical Damage character (and vice-versa) for that moment where you can only that one type of CC.

Synergy is the key Synergy between your characters but also within each character's build will get you anywhere. If you feel your characters lack synergy try to figure out why, sometimes it can be because of their Initiative Score (you often want to put Poison before Fire for example), sometimes it can be because they are doing too much of the same, sometimes it's another reason entirely. On a team-scale try to make the count of how many CC you have, their Armor type and which character holds them, the count of your defensive options, what mobility options each party member has. On a character-scale make sure that your damage-dealer character have good skills to get rid of armors fast and decent CC options that are cheap enough to use and with cooldowns that allow them to be used when needed to make sure you keep the upper-hand.

Creativity is your best friend.