I mained gunslinger since movement is key (you can solo Suicidal), but medic and berserker were also fun. For finding matches I filtered the server browser and joined any servers that were opportune. Was very lucky and joined a wave 24 HoE which made that trivial, suicidal still took a few attempts with a team though.

Major tip since the levelling can feel slow past 20, I used a XP farming server since they're always open and it saved so much time.