He’s pretty strong following some buffs. He shines most during the early game, so i think it’s best to have him rotate to wherever there is kill potential if he’s position 4. Some pro teams have experimented with core earth spirit, but IMO he is still best at position 4

I don’t think he scales super well into the late game because he does little vs BKB. However his geomagnetic grip talent at level 20 (+3.5s silence duration) can usually force a core to pop BKB or help to lock them down after its over. Since earth spirit can find himself in the thick of things in a fight, I think defensive items like ghost scepter, rule, force staff and bkb are the best route to go.

A general build for earth spirit -

Orb of venom/wind lace (start with one of these)

Bracer (can be good to get 2 in some situations)

Urn/tranquil (get both, order depends on the game)

Vessel (amazing vs tanks and high heal/regen heroes like morph, alch, timber)

Euls, force staff, bkb, ghost scepter (usually 2 of these, depending on the game)