OK, this thing is really rare, shrine sparks don't bring them and the number of fields having these shrines is a low number... Nevertheless I checked what brings more advantage: enhancing grade 1 gems or enhancing the high-grade killgem... Checked with grade 60 (32s, 64c), 1.500.000 hits, trap, enhanced with 5k monsters g60 enhanced: max. damage 8.9E19 g1 enhanced: max damage 3.3E21 So, when you are on a field having GES, use it with grade 1 gems and build the killgem with copies of these gems (I enhanced black and yellow...)
Enhancing Black and Yellow should get you one heck of a killing gem.
Enhancing Black and Orange, on the other hand, should get you one heck of a manaleeching gem. The real question is ... at what point should you stop enhancing the G1's, stop using the old managem, and make a new one?
The specials (crit, manaleech) aren't enhanced, so I'd suggest to use the whole period of permafreezing for using the shrine - the shrine charging times will go up to some thousands of seconds.... Idiotically I pressed 'x' instead of 'd', when I wanted to copy the more powerful killgem... And no, I won't restart the round, currently g75 managem (and two 8-gem combos of it) And: I managed to amplify a trap with seven(sic!) amps while manafarming