Impossible wither fight

Me and my friends were playing minecraft on their realm, I was on windows 10 they were on Xbox 1 and mobile, we summoned a wither underground, it broke through and started flying, we chased until y level 800 and ran out of fireworks, then a couple days later we fight another wither and defeat it, suddenly this one starts coming down, fires a couple of shots then teleports upwards, any attempt to hit it, it goes up. It’s coming down every 40 seconds and destroys some things, it appears to be moving as the skulls are appearing in.a general direction and it’s heading for our base, how can we beat it?