I trying to level all elements to 40 and only thunder is 40 so far rest are 15 10 ish so where should i farm?

If you're looking specifically for element exp, the more battles that are in the map the more element exp you'll gain. In Ohbanahara's Calamity Plains, the last one (Whispers of Death) has 5 battles and gives 90 element exp if you're taking 3 units of the same element for 16 energy. The map gives 720 summoner exp as well, so its not even that bad for that either. To put it shorter, look for the most battles that requires the fewest energy and that will give you your best element exp per energy ratio.

To add to this; EXP is based off of two factors: number of battles in a quest (multiplied by a map constant), and number of subunits of the same element as summoner (multiplied by another map constant). Bectas battles are worth 3 EXP each, units are worth 4, so a 4-battle quest with 3 subunits is worth (4x3) + (3x4) = 24 EXP. Vilanciel battles are worth 6 EXP apiece (so 24 base for most relevant quests), and units are worth 8 EXP each. Ohbanahara is 9 EXP per battle and 12 per unit. Thus it's actually very easy to calculate ideal EXP, and as Hhb noted, the only 5 battle quest (3-4) is incredibly efficient; (5x9) + (3x12) = 81 EXP a run, or 5.0625 EXP per energy - compare this to say 8-2, which is 16 en, 4 battles; (4x9) + (3x12) = 72 EXP, 4.5 EXP/EN; thus 3-4 is about 12.5% more efficient EXP wise. HOWEVER, 3-3's significantly cheaper EN cost means that it is actually superior for EXP at roughly 5.54 EXP/EN, along with far higher ST/EN.