May i ask how many ascension material do you have?
Tabards
Trap Tools.
Damascus
Tome
Compass
Fine Gloves
My advise is that you should work on some of your dark 4☆ before thinking in Quintus because they are going to be a cheaper investment to improve your gameplay overall(titan / raids / quest / war)
I think @Lexxtarc is giving you sound advice here. Concentrate on maxing Tiburtus and Sabina before you contemplate maxing Quintus. Tiburtus defence down and Sabina healing and cleansing are very important elements to have and you will likely level both by the time it would take to level Quintus.
I have Quintus sitting unlevelled and while I like using slow heroes for their power in a balanced Fast team and their ability to hit hard, I think there are likely to be better options in the future.
I have maxed costumed Quintus on 9th level of emblems. He has already 843 power and 834 attack. He also decrease enemy attack by 34%. I must say that costumed one is really worth to ascend.
I also have costumed Quintus and brought him to 15 levels of emblems. Both versions have a very high attack value which is great for Titans and to strengthen your tiles when you attack yellow tanks.
I am cheap to play. My three stack is Sartana +9, Rigard +18 and c Quintus + 15. They get rid of most yellow tanks easily. Sometimes I use Domitia instead of Rigard, when I have a suitable healer in my other color.
Yes Quintus is slow, but normally he only needs 3 more tiles than Sartana, which can happen quick.
The further up the ladder you go and the stronger the opponens the less effective rainbow becomes. In general. Some people swear by it, but the majority would not use rainbow past a certain opponent level.
I think 3-2 is a good comrpromise, and I say this after going exlusively mono for a long time. Now I do mono for my war hits and for all raid hits that don’t have a green tank - for Tel tanks (75% of my raids) I use a 3-2 red/purple that gets me around an 80% win rate. You very rarely have an unwinnable board with 3-2, as long as your hero matchings are well thought out
ameona and jabbar are not good heroes but Proteus is one of the best ones in the game. I would advice to wait for sartana or domitia . keep your tabards.
Tiburtus (normal/costume) will be more useful to you (raids, titans, events ) than Quintus (weak defense and no special effects ) unless he is surrounded by a good 5* team.
Rainbow team will not be efficient in diamond cup (except if you can align powerful heroes like gazelle, ariel, kageburado etc ).
Like @Homaclese said, a 3-2 attack team is the the best choice for you. You have to compensate the fact you don’t have strong 5* with high tile damage and multiple heroes charging their special.
Any 4* that give him support. Healers, def up heroes like Kiril or C.Melendor. the idea is that Quintus could survive to fire. If he fires he would eventually make serious damage. I dont remember now if there are others 4* that can boost the mana besides Brynhild.
If you can get tiburtus costume you have the best mate to Quintus. Because tibs gives def down to all enemy team in consequence Quintus will hit much harder.
So a good team could be C.Melendor-Quintus-Brynhild-C.Tiburtus-Proteus. As an example. There are lots of combinations