I've been playing this as well, started a few days after launch I think. Been playing for about six weeks.
I can't quite break into the top 100 arena ranking, though I've been as close as 111. I'm wondering if I'd do better waiting until closer to the arena day closing and doing all of my attacks then (15 min apart). I usually do 2 at Midnight when the daily objectives reset.
I played SW:GoH for a while and enjoyed that one. Lost interest sometime right around when they were adding the raids.
I didn't buy Captain America for $10 on day one, and wish I had. Still haven't quite unlocked him. As for spending money, I bought to gold package that was offered when I hit level 50, cost 20 bucks (roughly). Was really feeling the gold-pinch at the time, and was about to unlock Thanos, so bought the gold to catch Thanos up quickly. I'm still struggling on the gold front. I just hit level 57 and my entire A team is level 55. Spreading my resources a little thin, I guess.
One thing that that is frustrating about this game is that everyone uses the same characters. I'm guilty of it as well. The only character I use that I don't see much is Kingpin. I just got lucky early and got him out of premium orbs. Otherwise it's all Captain America, Yondu, Quake, Drax, Gamora, Crossbones, and some use Hawkeye. I really don't understand everyone using Crossbones. I ditched him as soon as I got Kingpin. Every time I go to the Arena it's just kill the Crossbones and then kill the rest of the team. If they've got Drax then I attack him three times (Gamora, Hawkeye, and then Yondu dispels his taunt), then kill Crossbones.
My arena team is: Gamora, Hawkeye, Yondu, Quake, Kingping. I use Thanos instead of Kingpin on defense. Working on getting Drax leveled and caught up, but I didn't start spending arena credits on him until after I got Quake to 4 star. I want to run Drax and Thanos on defense, though I'm not sure who I'll sub out. Probably Quake.
Are you guys placing in top 500 on blitz? Best I've done is 1-10%, which is where I regularly finish now. I really only have two effective blitz teams. All my other characters are too spread out on power-level to make good teams with. I figure once I hit level 60 I can start spreading the resources around some and make third and fourth teams that work well.