What fruits would be appropriate for different monsters?
Here's a brief breakdown.
Monsters generally have an inclination on their growth towards certain stats. For instance, Mangrodons generally have an inclination to grow towards SPD. Hence, SPD fruits will give around 0.8 - 1.0 stat boost for SPD.
Monsters that do not have inclination towards a certain stat will have around 0.5 stat boost for every fruit fed for that particular stat.
So, do you use Fruits to further boost their inclination? It's YES and NO.
YES - When your monster has inclination towards desired stats such as S.ATK/ATK that will provide a greater output damage in battle.
NO - When your monster has inclination towards stats that are not necessarily needed due to its nature. (E.g Averagely, Mangrodon will have about 130-140 SPD at level 50 without having the need to feed SPD fruit) This SPD value is sufficient to allow Mangrodons to perform an action first before most opponents (Including Krakrows). Hence, other fruits can be used to compensate for the low growth of other stats (Eg. S.DEF/ATK/HP)
Is DEF fruit needed for Teslith/Lumenore since DEF is a fundamental parameter for Teslith's/Lumenore's main skill, BLOCK?
DEF is very dependent for Tesliths/Lumenores, but it is also noted that Tesliths have an inclination to grow towards DEF, hence even without DEF fruits, Tesliths/Lumenores will develop close to 120-130 DEF at level 50.
Since Teslith/Lumenore is a Ground type monster, Teslith/Lumenore generally have the most ideal body type for BLOCK since almost all physical attacks of the current Little Masters' monsters do not have high effectiveness against Ground types. At 120 DEF, most physical attacks will still perform 0 damage to Teslith/Lumenore regardless whether its a critical hit or not. Hence, DEF fruits are not needed. In fact, not needed at ALL. However, in some cases, DEF fruits help to spur the growth for the DEF parameter, hence an adequate amount of DEF fruits would be sufficient for a good BLOCK built Teslith/Lumenore.
Having 120 DEF and 170 DEF for a Teslith/Lumenore does not make much difference since the damage output required to KO an opponent is averaged about 220 damage. Hence, 2 rounds are still needed to succesfully take down an opponent assuming opponents do not switch or use any physical attacks after the first BLOCK hit.
Teslith's/Lumenore's greatest weakeness is Fire based attacks. As of now, there's no physical attack that is Fire natured, hence BLOCK is still an effective skill to counter Physical Attacks. However, a mere Burn/Ignition/Exposed Flame/Burst is sufficient to take down a Teslith/Lumenore in just 1 round.
Based on the scenario above, the only known way to increase resistance towards Fire based attacks are to increase S.DEF. That's when S.DEF fruits come in handy. High S.DEF does not mean that Teslith/Lumenore are shielded against Fire based attacks, but simply providing a greater resistance to Fire based damage over time and even Fire based attacks, allowing Teslith/Lumenore to utilize its Level 12 Heat Conductor ability efficiently. With a resistance to being 1-hit KOed by Fire based attacks, Teslith/Lumenore will in turn gain 2 levels of ATK and perform a successive Smash that deals good damage to an opponent.
Hence, a balanced Fruits distribution for Teslith/Lumenore would be
S.ATK fruits may come in handy for Lightning at Level 32. Teslith/Lumenore is also known to have an inclination towards S.ATK, hence allowing the growth of S.ATK to be slightly boosted.