I think their point is that what you've given is a pretty miserable setup, and so for the most part, as far as developing a better bot, it's best to scrap every part you have there and start from scratch, basically meaning all we have is a BD to work with.
Now, I'm not all that familiar with Gathima BD2 (I've never even seen a Gathima in CB yet). I have no idea how well its weapon works as anti-air in practice, so everything I write from here on out is purely numerics and slightly educated guessing.
Looking at its capabilities in the calc, it probably has major capa issues since it has very weak capa carts (+50/15). Compare Psycho Formula, who has the same capa carts, but doesn't have to worry about EIW, costs less and has more capa to begin with (among several other nice things). Significantly, he also doesn't get Short Boost, which doesn't really help for MB builds.
EIW does seem to be the first step - both carts give you 4 force, 60 ammo, and 105 speed (topping out at 470, which ain't no railgun but still fast enough to hit fast Airs if you aim it right, I would think). Given all this, it seems like you really can't support a proper alpha build (no Short Boost, capa issues), so if your goal is anti-air, I'd suggest trying out a mono-weapon build focused on his internal. Breakers can be general all-range type of weapons, so you don't really need a "back up weapon" to cover its weaknesses unless you are processing a longer-range weapon, in which case, the breakers end up as your backup.
Swap that awful BS for something nicer (Maril BS is a good start), and I won't feign any expertise in any non-BS recommendations for M Arts, but general direction being TEC and FLY. I have no idea what your library of parts is, but start from there (again, focused on TEC and FLY), practice with the weapon in matches. Weak cart choice, poor capa, and not exactly the best of stats gives little benefit for the core other than this internal - so if you want this to work, build around that internal to maximize its potential. On the plus side, at least you won't be burning your low capa on weapon tunes.