For an ascended shotgun 200+ damage for high 145-150, 5 shots will do to bloody it.

Unfortunately on some servers the lag will require to wait for those shots to register with the acro. Because of this I used a tanky deinotherium(high drag weight), health pumped to about 40k with a solid saddle blueprint to eat the acros damage and keep its focus in passive.

Position yourself far enough behind the acro and shoot its rear. Once the required shots have been made, wait for it to leave shield stance and it will 99 percent always go into its adrenaline mode. Run up behind it and feed the bio toxin then step away. Rinse n repeat. This method also removes the fake roar it does.

If this seems too much I think some ppl are using the managarmr method.