Honestly, the only thing I would do is....boost its burrow speed to 20.
With that...the ankheg already has everything it needs, because burrow is awesome. Remember the tunnler feature only is needed for solid rock, otherwise the Ankheg can just burrow under the ground with no trail left as normal for a burrow speed.
1) Grapple a target and pull it under. Congrats its now a 1 on 1 fight.
2) With its 60 ft tremorsense, it can move to an area the party is not, pop its head up, and release its acid. Party might get in some ranged attacks through readied actions, but feel free to give it solid cover for only popping its head up.
3) Perfect hit and run animal. With disengage + burrow, it should often get away. Combine it with some healing thing (perhaps an underground spring with healing powers) and the creature keeps coming back for more.
With that...the ankheg already has everything it needs, because burrow is awesome. Remember the tunnler feature only is needed for solid rock, otherwise the Ankheg can just burrow under the ground with no trail left as normal for a burrow speed.
1) Grapple a target and pull it under. Congrats its now a 1 on 1 fight.
2) With its 60 ft tremorsense, it can move to an area the party is not, pop its head up, and release its acid. Party might get in some ranged attacks through readied actions, but feel free to give it solid cover for only popping its head up.
3) Perfect hit and run animal. With disengage + burrow, it should often get away. Combine it with some healing thing (perhaps an underground spring with healing powers) and the creature keeps coming back for more.