How to be decent...

Karel is really good imho and he can achieve that even without inheritance, so that's a plus for a new player. He's very weak to any ranged characters, especially mages, from my experience, so a mage killer is always welcome (some bow wielders, kagero, fliers). Other thing that I can recommend for every team is a dancer/singer, but you should wait for azura/ninian rather than investing in Olivia. Then you could get a magic/physical dmg bait in your team, because Karel has a lot of hp, but he has low defenses, so you risk having him die to the most common arena mages (which are blue).
You should be able to beat most of the common teams with this. Karel kills Hector, your mage killer kills their blue mage (mostly blue - Linde, Reinhardt) and your tank can take hits from takumi or other ranged units.

I'm not a super experienced player myself tho. I'm rank 15 in arena, but I'm a f2p player who doesn't get access to the whole roster and I also can't really make the strongest heroes through inheritance. What works for me tho is Zephiel as a physical dmg tank, Camilla as a blue killer, linde as a glass cannon who one shots almost every red unit and usually severely damages anything else, and lastly Azura/Ninian that I switch frequently, because they both have their significant advantages.
Hope it helped.

Make sure you pick up ursala on the GHB. Even if you can only manage the 3* version she is worth investing in.

The hero fest banner was a great banner to pick up a stud...it's unfortunate if you didn't pick up a decent hero on that banner.

Also look at investing in anna, sharena and alfonse. They're always bonus characters in arena so if you're f2p they're very useful to have levelled up to boost arena score.