With the right team mix and some sequence testing, it is absolutely doable. I've used a 36k team to beat 42-43k teams in Arena so squad power is not an absolute indicator and now that my arena team is at 40.8k, I'm getting mowed down by Wiggs team at 37-38k.
Lastly GW isn't a 1 team only thing as Toukai has elaborated. Before I got the easy node 12, node 9, 11, 12 and sometimes 6 were really hard. I used the sacrifice team tactic where on node 12, I would use a secondary team whose sole job was to kill 1 off so that when my main team came in, it was 4 vs 5.
If your team is not HK lead, then it's pretty much destined to lose with what you have described. Lumi lead is pretty useless now. BTW my 36k team that was beating 42k teams in arena was, HK lead, 88, 86, RG, Rey. All F2P toons