Here is my thoughts on about Alchemical Upgrades for situationals :

Best DPS imho for all purposes. Yes mostly devil bosses immune to fire but weapons can bypass their dr and you have provide +10 Seeker and +2 Str. This one is very good for epic trash imho. If you have twf you want to always use this.

Base : Silver
Upgrade 1 : Fire ( +5 Enchantment and +10 Seeker )
Upgrade 2 : Fire ( +2 Strength and Rigtheous )
Upgrade 3 : Air
Red Slot : Adamantine or +7 enchantment

Second Choice Maybe ( You loose 2 str but gain good dps )

Base : Flametouched
Upgrade 1 : Fire ( +5 Enchantment and +10 Seeker )
Upgrade 2 : Air ( Lit and % 6 Doublestrike )
Upgrade 3 : Air
Red Slot : +7 enchantment

Anyway please share your thoughts for upgrading your alchemicals.