They aren't extremely fast and are easily outrun, so just shoot it with a crossbow using tranq arrows and turn around and runa. Few steps when it gets too close. ( may be best to take out any other mammoths before attempting to tame )
The mammoth already has a platform saddle in ark mobile but you have to buy the engram with real money. Click the up arrow if you want to help convince wildcard to add it into all versions of ark fully including ark mobile for free!!! Help start the petition revolution!!!!