So as some already said, classvariety has a huge outcome to scenarios. If you have only 1 healer for 12 people and enemy team has 4, your side already has a big disadvantage. Then there are certain scenarios which benefit a certain playstyle. Lets take Gate of Ekrurnd as an example. If your team is low on tanks and mdps but you play a mdps you will have a very hard time cause you will draw a lot of focus on you every time you try to capture the mid flag. If the enemy team has a lot of range dps and you have a lot of tanks and mpds they might have a hard time cause your team rushes them down.
Split classes even in a scneario. You can switch groups in scenarios, so if party 1 has 4 healers and 2 tanks and party 2 has 6 dps, try to get 2 heals and 1 tank per party.
Buff your party at scenario start.
General tip for beginners:
Try to keep your gear updated, (eg as lvl 20 dont use lvl 10 armor), if you cant afford the warcrest weapons and armor, atleast get the one for gold from merchants. try to slot highes talismans you can get; tanks and healers (healers try to get tanky in this game) should use toughness, wounds or armor, mdps strenght, range dps intelligence or ballistic. With more experience people might change stuff, like tanks going for strength and some dps stacking weaponskill, but for leveling this should be fine.
Tanks should use Guard (even if you use a 2handed weapon) on mpds, and switch guard if range dps or heal get attacked and stay in Guard RANGE, never use Focus Offensive Tactic.
Guardpack addon draws a colored line between you and your guard target. If the line is green you are in range, this is very usefull to follow your target.
https://tools.idrinth.de/addons/guardpack/
Healers: Dont heal by clicking on the playeymodel, in fights you might miss them several time. Use the party overview to monitor they hp status and and to select your heal target.
Enemy Addon is very good to improve the visibility and help you with that. In fights heal>revive, only revive if the rest of you party is save/full hp, usually you focus on your party first, if your party is save/full hp you can heal outside of you party
Dps should always try to focus on same target for fast kills. If a target is healed and guarded 1 dps will never be albe to kill that target. NEVER
If you get focused use your detaunt and try to escape. If you barely survive or die and get rezzed, dont rush in again, give you healer time to recover or you will die instant again.
Enemy is probably the best overall addon for this game, use it.
https://tools.idrinth.de/addons/enemy/ It allows you to mark people, like your healers, leaders to follow etc, it gives you a better party/warband overview, you can use clickcasting, and a lot more