I just stay at HK range and harass people with it since it’s an easy hit confirm and it’s relatively safe. It’s -2 but because of it’s long range no one can punish it (well Aria can with her 1 frame shadow DP but that’s the fastest move in the game) or start pressuring you after blocking it. I’ve also found that Rash is a great character for a reactionary gameplan as opposed to a rushdown pressure gameplan. His HK has ridiculous reach and moves him forward so it’s a great whiff punish button. You can also make people never want to jump if your anti-air game is on point. He has four ways to hit people out of the air: crouching HP, big boots, tongue, and wrecking ball, and all four of those lead to full combos. So if someone jumps at me, crouching HP into full combo. If someone jumps at mid range, heavy boot into full combo (or use standing HK to try and catch their trip guard) if someone is jumping at full screen, quick hop into tongue into full combo. The only place Rash can’t hit people out of the air is directly above him, so corner jump pressure from characters like Sadira and Maya eats Rash alive.
As for wrecking ball my favorite use for it is after a wallsplat. The medium and heavy versions have enough start up to where they won’t combo, but they’ll avoid and hit grab tech attempts, they’ll catch people out of the air if they attempt to jump out of the corner, the armor will kick in by the time they recover so it’ll soak up and hit them if they try to use a reversal to get out of it, and if they block since you’re in the corner you can actually stuff non-invincible anti-air attempts with the medium version followed by an MK on the average to tall characters in the cast. Short characters can still anti-air you before the follow up attack.
TLDR version: Every time the opponent jumps anywhere on the screen Rash gets a full combo, so stay at HK range and annoy people with it’s reach and safeness until they either get hit by it or they try to jump in. Also wrecking ball after wallsplat is pretty good.