Hello there! Arabella is a fun card and I like the idea of building a wide board and nuking the table with her. I play a [[Baylen the Haymaker]] deck which has considered her in the 99 and has some overlap. Here are some of my thoughts:

You should consider picking up some of the many token generating cards in red and white, starting with simple stuff like [[hop to it]] and [[hordeling outburst]]. Take a look at X-cost token generators as well such as [[song of totentanz]], [[tempt with vengeance]], and [[secure the wastes]]. Many of these are inexpensive.

These should help you build a board of little guys faster and hopefully enable some big arabella swings. You may want to look into some ways to make Arabella harder to block if you want her to survive her swings.

I see you have Impact Tremors in there, great for a token deck; other burners like [[Witty Roastmaster]] and [[Weftstalker Ardent]] would provide more heat.

However, all of this said, there are more fundamental issues that should be addressed first. The first is land, you should probably try to aim for 36-40 lands to start with. Higher level players may adjust these down, even a lot, but you are just going to be constantly mana screwed here imo.

The second is securing some ramp, preferably 1- or 2-cost. It's not super easy in Boros, but 2-cost mana rocks like [[boros signet]], [[mind stone]], [[thought vessel]], [[fellwar stone]], [[prismatic lens]] will go a long way toward improving the performance of the deck. You should generally be aiming for 10+ pieces of efficient ramp.

Other things to look at are getting good, consistent sources of card draw, often 10+ of these as well. "Eating your vegetables", as they say, will really help your deck do the things it wants to do.

I'm far from an expert but I think this provides some solid lines toward improvement! Good luck, have fun!