Hiya @Yanks1111, welcome to the forum!
Firrst off, materials come with time 
it’s a bit of a spiral. To get better mats you need better heroes to score better on titans etc… and get better mats to make better heroes
On the flip side, it can be a spiral upwards. As you level more heroes, you do better & deal more damage so you get more materials so you get better heroes so you deal more damage to get more mats etc…
So the outcome of all that is to be patient. The mats will come
In terms of levelling advice, I would suggest not focusing on your 5* heroes until you have a firm 4* hero foundation. Reason being is that 5* heroes are both expensive and time consuming… They soak up precious resources, feeders, experience and time.
Another reason to focus on 4* heroes is that a maxed 4* is better than a 5* stuck at 3-70… They’re typically stronger & can be emblemed immediately!
So what that looks like in reality (IMO) is to focus on 4* heroes until you have 4 rainbow squads of 4* heroes maxed… (i.e. 4x maxed 4* heroes in each of the 5 colours for 20 maxed 4* in total). By this stage you should be well into the rhythm of killing titans etc… & will know what your play style is, which style of heroes best suit it.
So back to your OP question, I would suggest you use the following defence:
Sonya → BT → Li Xiu → Cyprian → Melendor
It’s not a great line-up but its solid. These would also be my suggested priorities in levelling up your heroes:
Red - Boldtusk first. Then probably focus on Scalett or Colen (big attack stats)
Blue - Sonya then Grimm then Kiril.
Green - Melendor first, then buddy. Leave Tarlak alone for a while yet.
Yellow - Li Xiu then Wu Kong then Chao
Purple - Cyprian. I’d then possibly push Grimble to 2-60 but not to 3-70 yet (unless no other new 4* purples come along)
In terms of why I suggest not worrying about Tarlak just yet is because:
a) he’s mat locked… His sturdy shields will be better spent on other 4* heroes than tarlak for the above reasons
b) He’s actually not really an improvement on Wu Kong… Definitly not at 2-60 and unlikely at 3-70… Check the link below for comparison of Tarlak to Wu Kong & why I am saying this.