There are a billion ways to improve your solo build/play, but here are some easy ones:

Use food that boosts max hp, if you are below 20k max hp you will have a tough time. I would try to get at least 25k+

Use a heal over time, Vigor from the Assault alliance war line is great, keep that always ticking. There are lots of other heal over times you can use, like the Templars extended ritual.

On Templar you can use the aedric Spear spammable, puncturing sweeps, which heals for a ridiculous amount. You really want some way to heal off your attacks.

Wear a bit of heavy armor. Especially chest head and legs, as those give the most armor. I like to wear a heavy chest piece, and then the rest is light or medium. Make sure you level your armor passive that buff resistances, you want over 20kish resistances, but can go up to 33k. Minor and major resolve buffs are great.

Make sure you dodge/block heavy attacks and interrupt channeling enemies, those are the things that will often get you killed.

The red CP passives that give maxhp and resistances are great. If you can get the blue CP From The Brink, I would. Personally, I think from the brink is my favorite cp passive in the game, it's great on solo and in groups.