This is a simple guide to 5,000 combo in BA, you can use a different strategy but this worked easily for me. (Works best with a gun, because guns have fast speed)
Before getting TOO strong, go to an area with a boss that only melees (I used the Old Snail in Mystic Forest because it has no melee and heals). and blast your weapon until it overheats. (Overheat decreases damage) Once you encounter the boss with an overheated before you should be doing 1 damage a hit, then just sit there and fire, using 'Quick' skill also helps the combo go faster.
Thanks for the feedback !