What you're trying to remove is the barrel nut, the delta ring is just a stepped sleeve around the barrel nut that is mostly only found on older style guns (usually fitted with plastic handguards) and it doesn't hold the barrel on - almost as common a myth as the ukara license. Barrel nuts are often torqued on crazy tight in airsoft, frequently tighter than is needed even on a real AR-15, then they even glue the buggers on sometimes. Also there's occasionally weird ways the nut might be locked on, grub screws, or it could even be fake in some rare cases and the barrel held on in some strange way, that's the nature of airsoft.
Speaking generally: Depending on brand, a strong vice, heat gun and (as demonstrated) a really sturdy barrel nut spanner are very much advised. Tools are nearly as good of an investment as eye pro when it comes to putting money in to stuff you can use your entire life.