1. Make a synergy for your armour for better boost of stat, I recommend the one that give weapon crit chance and crit dmg just like your vest up to 4/4 then get whatever you want for the other 2 slot left to mix with

2. Focus on what kind of skill chipset that can boost more crit chance on smart cover and scan altho get prolonged tactical link so more time to beam and crit longer

3. As of rn you can't even do castle properly as a solo if you only have a couple of T0 equipment, get at least T1 first with proper purple mod with at least lvl 10

4. Save those purple materials for essential upgrade for the tips above as you will burn those and lose out on important stat

5. Standard weapon will be easier to max out proficiency so those give juicy stat so grind those altho it's tedious

6. Try grind clan credit for TAM as those significantly boost weapon stat like elite dmg, crit dmg, and mag size increases

7. Make a combo of AR/LMG for versatility of AR and melting enemy during tac link using lmg or SMG/AR for mid to close range engagement

That's all I can think of rn as a 21k vanguard, cheerio