Strategy Notes

Core

The game seems to be 'melee unfriendly': if you want to use a melee character, you will get al kinds of slows, knockdowns, hexes and conditions that limits what you can do during combat. It takes some time before reaching enemies and then you are vulnerable and output limited damage due to conditions.

It would be logical to combine a ranger with a warrior as second profession. You basically only use ranger skills, except for Frenzy. You could invest a lot of attribute points in markmanship and some in surival. If I remember correctly, some points can still be added to beast mastery. Furthermore:

When making a good build, avoid letting it do 'a little bit of everything'. That is usually a bad decision. Instead, try to find a good elite skill and add some attributes. Build the rest of your build aronud those two things. When going for damage output, don't only look at base damage. Other things like cooldown could be relevant too. Some skills could be more useful for damage output because you can cast it more often. Also, try to minimize use of build resources. A rotation that takes a lot of time to build up, isn't usually very useful.

When constructing a build your always looking for things that are high in synergy, high consistency and low in redundancy so that the result is greater than the sum of it's parts.

As an example, Water Trident has high damage and low cooldown, which means you don't necessarily need many additional damage spells to achieve consistency. Either they need to do even more damage so they're worth casting over another trident or provide something else that trident doesn't. And by leaning into that second option you will find a lot of room for more utilities Water spells, which is also the reason to pick that Element at all over the otherwise higher damage Fire High synergy, low redundancy.

If you instead pick Mind Freeze then there's a longer cooldown period to cover and additional conditions to meet to use it at max capacity, meaning it will automatically occupy more of your build resources. If you're goal is to make mind freeze work then that'd be fine, but if it's something else then that's a reason to reconsider Less synergy (depending on the goal) and less consistency.

About evaluating battles:

It's kind of difficult, but if you're using hero healers, keeping an eye on their energy does help. Support builds like water magic always make it difficult to see what your impact on the battlefield is, but if your team isn't suffering its first casualty within seconds it's doing something right. Generally if you have Aegis and Prot Spirit on a protter, the rest of it will be fine as long as they're not all 10+ energy, and energy management on your protter is practically a must have.

In general it's pretty hard yeah and it's often just easier to go by trial and error and experiment. A thing you can keep an eye on is what resources are running out how quickly and how often.
Prot is most effective against fast spikes of damage, while healing counteracts more consistent pressure. So if you find that your getting chunked down quickly and your healthbar is on a rollercoaster, it's probably the prot.
If you find your healthbar is going down slow but steadily and not really recovering, it's probably the healing.
If you find that your in long engagements often and your health is mostly fine, but you struggle to deal with enemy healers then it's probably the damage output.
There's also Energy economy but that's overal less of a team comp question and more of a consistency problem for the individual build (aside from "hey should I have someone with Blood is Power?").

PvP

Fort Aspenwood

If you wanna start with Fort Aspenwood, a good starting point would be to use fire magic and do AoE damage.
Usually, you can use a build that works in PvE with some tweaks to make it usuable in PvP.

Necromancer

Spoil Victor is a good anti-healing spell.

Prophecies

Presearing needs no special strategy.

When facing those flying octopus things in Ascalon, remember that these can deal a lot of damage with a hex. So do pay attention to hexes applied to you.

In Northern Shiverpeaks, remember to kill healers first and then other casters.

In Kryta, kill healers again and note that imps give a lot of damage and set on fire. When in Lion's Arch, it is easy to do quest to start with EOTN. Helps getting heroes. Once in EOTN, attributes increase based on level. So EOTN can still be played even when you're not level 20 yet.
When wanting to kill Galrath, travel to the Temple of Ages first. You can then have a larger party to make it easier.

Maguuma has no special strategy. But in the Henge of Denravi, there is a weapon seller that sells good offhands for casters.

In the Crystal Desert, the hydras deal a lot of damage and cause knockdown. You can also get knockdowns and hexes, so it is important here to have a diverse party.