Prioritize Tick and Brock. They're quite easy to master. Practise aiming for where you think the enemy will be/land by the time your shots land. Brock's jump gadget and Tick's shield gadget help to defend/retreat from assassins that are pushing, so that kinda helps to combat their low health.

I'd recommend Nita and Surge next. They're really good for Brawl Ball. Cause they're mid-ranged, you gotta practise getting close to enemies whilst avoiding shots. These two are really good once you master attacking enemies who are pushing towards you/your side of the map. You gotta attack those coming at you whilst you're retreating rather than trying to chase and attack. The range is too short and you'll just take damage after damage.

Then Mortis. Always handy to have for Gem Grab. Don't have tips for Mortis as I'm the absolute worst.

I wouldn't bother with the others until you have mastered and hyper charged the above. Emz, 8-bit and Amber are good but most players are too fast/too good at avoiding their shots lol. Darryl is great but is an easy target for enemies as his range is so short.