Spending 100 catalyst on an "empty" totem will still have a chance for a random skill. Therefore use 10 catalyst once, choose a skill and keep it, no matter what you get. After that, you are able to choose the skill you want when you spend at least 90 catalyst - you are guaranteed to get that one!

Always spend 100 catalysts because it offers by far the best chances to get the the higher skill levels 4 and 5.

EXCEPTION:

The only exception to this rule is when: 

• You are getting close to finishing a Crystallized Might (CM) - only around 600-700 catalyst away (i.e. you are at 2.300-2.400 / 3.000)
• AND You have no rank 4 skill right now that you could improve to rank 5 with the CM

In that case use x10 catalyst until you hit rank 4 that you can then upgrade with the CM. Getting to rank 4 that way is very cost-efficient!

Note that CM allows you do choose any skill, so it doesnt matter what skill you get with the rank 4 upgrade. Also note that you can delay switching to the rank 4 upgrade until you have CM ready, if the skill you got isnt the one you want! So you can keep using your current skill until you finish the CM.

Do not use this method until your skill is on rank 5!

On rank 5 spend 10 catalyst per try and decline the result until you get the new chosen skill on rank 5 - should take 12-15 tries and cost 120-150 catalyst (on average).

Important: NEVER take a rank downgrade from 5 - It would be a lot more expensive to get back to 5 than what you have to  spend on the skill change!

Use BLP on rank 4 skills only - ideally Elemental skills, as they are better (check #6). Make sure you have a lvl 4 skill ready when you get close to 100% on the BLP counter - you cannot stock up on BLP!

Check #2 (Exception) when you are about to finish a "CM"

I don't have hard facts to present here. But it doesn't require much testing to see that elemental skills have a lot higher effect on the battles than the primal skills that sometimes make you wonder if they actually do anything. Therefore, they should be prioritized when using the Crystallized Mights (BLP).

A few exceptions where primal skills are actually noticable include the strong synergy between Triple Cycle and Echo Aegis, esp. with Avalon in the team to spam shields that trigger both primal skills. Also noteworthy is Primal Zeal as a whole, both for the RNG factor on defense, but also because it pushes titan activations from the energy gain. It also can turn Sigurd invincible (with a little luck).

Flame Dance, Dephts Whisper and Murmur of the Crags have a 200% damage/heal increase from rank 1 to 5 (on otherwise maxed totems). They scale a lot better than the other elemental fusion skills and should therefore be prioritized.

Last Flashs attack buff does not scale from the fusion rank - it is based on totem level and evolution. But the duration is increased by 240% between ranks 1 and 5. Which also matters a lot, esp. when you use Nova as the tank. It makes Last Flash another skill to prioritize highly.

Ice Age has a fix stun duration and damage buff (based on totem level/evolution), but the duration is increased by only 160% from rank 1 to 5. I.e. only 2/3rd of the scaling of Last Flash!

Wrath of the Depths has the lowest scaling: the damage is fix (based on totem level/evolution) and the stun scales from 1.5 sec on rank 1 to 3.5 sec on rank 5 - only a 133% increase. It makes Wrath of the Dephts the least desirable skill to improve, but arguably also the best skill to use on a low level!

All three active elemental skills have a delay of 1 sec from the totems activation to their skill activating. From then on, all three skills have different durations:

Dephts Whisper (DW) has a 9 dec duration of dealing dmg and healing twice per second for 18 ticks total: 440k healing per tick shared among the team (=8m healing). The dmg is a bit weird and depends on enemy proximity it seems: the full 440k vs. nearby enemy tanks, but middle row takes less dmg (297k or 333k) and backrow seemingly none. So the longer the battle takes and the more tanks and less backline titans the enemy runs, the better is the dmg part, which can theoretically go up to 440k x 18 x 5 = 39m. Overall DW has the highest dmg shift (8m + up to 39m), but the longest duration, i.e. needs the most time to unleash it. Note that DW activations stack: if DW activates again while the previous one is still active (i.e. within the 9 sec duration), the new activation will not override the first one - instead both will be active simultaneously.

Flame Dance (FD) takes like an additional 3-4 sec to do the full dmg. It will usually hit each titan 4 times for a bit more than 3.5m dmg each, i.e. 18m total (on a full enemy team). It is the middle ground of the three skills both regarding "numbers" (dmg) and time to be effective. However there is a bug (or a feature?) of FD manifesting on an enemy Summoner like Phyto when it activates. It causes the FD to travel through the whole screen before it hits the enemy, which is a massive nerf.

Murmur of the Crags (MoC) is always a 15m dmg shift (5m shield + 10m reflected dmg). It needs only that original 1 sec delay to be active, i.e. it releases its full potential the quickest. Compared to the other two skills, it also scales with a decimated enemy line: while DW and FD do the same dmg per titan and thus overall less dmg when there are less enemies, MoC will do its full dmg vs. a single remaining titan. As a bonus, its shield has debuff protection vs. stuns etc.

Overall FD is by far the worst skill of the three, mainly because of the bug/nerf with enemy summoners, and shouldn't really be considered nowadays. The choice is between MoC that offers protection incl. debuffs, which is strongest vs. aggressive fire teams, and DW with the highest numbers, which is best in slower matchups - basically everyhing except fire. DW is also great on defense teams with CoW damage reducing buffs, because that gives it time to release its full potential.

We learned in #6 that Wraths of the Depths has the lowest scaling, just after Ice Age. Both skills offer a good damage component that even has a good synergy: for example when your enemy kills your 1st titan with his totem activation that also triggers your Ice Age - the solid Wraths of the Dephts damage will be enhanced by Ice Age and hits the complete enemy team.

Also both skills stuns are better than the short durations suggests: often a 1 sec stun costs the opponent more time than that, because a lot of titans have long casting times / animations for their ult - like Eden pulling up his rock or Nova doing a long stretch. If you stun them at the last moment, they already lost 2-3 sec from their animations.

For someone with high level / maxed titans and totems, who does not want to invest anymore, my suggestion for an impactful Titan team in GW, CoW and ToE would be:
3x Earth & 2x Light team with Ice Age and Wrath of the Depths - useful on ranks 1 already. Add Triple Cycle and Echo Aegis as primal skills.

Echo Aegis and Triple Cycle activate each other and thus forms a strong synergy that outshines any other combination of primal skills. It is imo the best combination esp. for the common Earth/Light teams, but likely any other Earth team as well.

It is however only (really) valid for Earth teams, because Echo Aegis relies on Avalon to activate. Rigels shield also activates it, but a lot less frequently.

Triple Cycle also activates a lot from Totems that heal, i.e. the basic Water Totem skill, but also Depths Whisper.