I was going to comment last night, but @Garanwyn did an awesome job breaking down and explaining everything, that I didn’t need to. 
As for leveling, I’d recommend holding off on the 5* for some time. The 4* are where I’d focus your attention, at least until you get a few leveled in each color. Although, both Onatel and Isarnia are amazing heroes, they take a lot to ascend and level- so you’ll finish two 4* in the time it takes to finish one 5*, and 4* will be stronger than 5* til 5* hit 4T.
(Since this isn’t a photo of your roster, it’s a little more difficult to get this accurate, but I’ll do my best)
Leveling:
Red: Boldtusk is amazing for everything in game, so I’d typically have you max him first out of your reds, but leveling Kiril and BT at the same time (if none of your other heroes are decently leveled) may have you lacking in enough offensive power. So I’d do Gormek first for his defense down ability, then Boldtusk. Colen for defensive purposes and the high attack stat (4*).
Blue: Kiril is just as amazing as Boldtusk, so I’d have you do him first for that buff and heal. Then, I’d do Boril I spose for the versatility, and some people value riposte (I don’t, but others have found it useful), so he may be of some help to you (4*). Isarnia I would wait on until you get Kiril and another 4* blue maxed and have no other blues (including 3*) to work on. Once you get the chance though, Isarnia is definitely worth it for her high attack stat and lowering defense on all (great for Titans) (5*).
Green: Buddy is a solid hero, and the defense down is always a nice skill for a hero to have, so he should come first. After, I’d do Little John for that high attack stat and lowered mana gen (4*).
Yellow: I’d actually get working on Chao first for the fast mana cut on one (better offensively), then Li Xiu for the average mana cut on all (better defensively). If you happen to pull Wu Kong, move him to the front of the line (4*). Onatel can be really fun to play with, especially to have mana control, so I’d work on her after you get a few other yellow 4* leveled first (5*).
Purple: Rigard is an amazing healer, and the only 4* with cleansing, so I would keep with him. After, I’d work on Tiburtus for another defense down in another color (nice to have several for war, Titans and events). After, I’d do Sabina for a deep heal and dispel since you have no other heroes with that ability, I’d maybe move her ahead of Tibs if it’s needed. Cyprian can wait, you’ll already have a riposte in Boril, so I’d leave him til last (4*).
Eventual Defense:
Tiburtus-Boril-Boldtusk-Buddy-Chao
Don’t like Tibs in the wings, but don’t see any other options. Don’t want to run two healers with BT and Kiril, so Boril is the only other option and he needs to flank or tank. Honestly, your best tank options (in 4*) are Boldtusk and Li Xiu. Onatel once she’s in 4T. You’ll want your most important heroes in flank to ensure they go off, and fast mana snipers are preferred in the wings as it doesn’t take much for them to charge. Al moves from left to right, so buffs, defense downs, cleanses and dispels should be on the left.
For attacking, you’ll just run whoever is your strongest rainbow until you can run more variety. Then, there’s color-stacking or tailoring your team to best counter your opponent. Impossible to know who works best without knowing who you’re fighting, but you’ll probably want one-two healers and 3-4 offensive heroes. Defense down + attack up is an awesome synergy.
For Titans, you’ll want to bring in as many heroes that are strong vs that Titan as possible. Always bring a healer/attack buffer and a defense down hero. Fill in the rest with strong or neutral colors with the highest attack stats (Titans are all about tile damage). Once you get Wu Kong, always bring him into every Titan fight for that tile buff.
Hope that helps. I’m here if there are any more questions left unanswered. Good luck on your leveling 
. Experiment, and learn how your heroes and your opponents heroes interact, and you’ll start to learn what works and what doesn’t.
I concur with the others about focusing on your 4* heroes and level the 5* as your mats allow. I believe when others talk about stacking colors, they’re talking specifically about defensive teams. In my opinion, it is bad to stack colors on attack teams because you sacrifice one of the color’s power as you attack. However, on a defensive team, you have the opportunity to focus more on the special abilities of your heroes. Double up on heals like Boldtusk, add some aoe (area of effect) damaging heroes, and add one strong (single target only, aka: sniper) hero. It would be a fairly impressive 4* defense team.
