Earth sense isn't that bad, since it's one of the easier/more reliable ways to keep from getting ambushed by invisible people at lower lvls.
Not ambushed. Can easily move and attack from more than 20' away, even at low levels.
For the purpose of tracking down invisible enemies you've already engaged with, however, it is useful.
As for early entry tricks, Earth Spell gets you in early, as does DMM: Heighten or Naenhoon, since the latter two effectively replace the adjustment instead of reducing it. More general metamagic reducers, however, are questionable (at best) as to whether or not they work with Heighten Spell at all because of the strange way the feat is worded. So, if you Heighten a cantrip to 1st-level with those 3 metamagic reductions from Metamagic School Focus, the reading of the feat suggests that it might still only count as a 1st-level spell. That's why people tend to favor Earth Spell, DMM: Heighten, and Naenhoon over Metamagic School Focus (in fact, the only person I *ever* see touting Metamagic School Focus is Endarire).