I don’t have the direct answers for you, as it’s kind of complicated and I haven’t been hitting low-mid Titans for a very long time. But I’ll do my best!
What I can tell you is it depends on your playstyle, what heroes you have available to you, if you use items, and where you want to be on the Titan leaderboard.
My best advice to you is to hit as high of a Titan as possible without breaking the bank on items (I’m talking expensive items, tornadoes/time stops, flasks). As long as you hit 1% of the Titan’s total health you will get full rewards. This adds extra stress on your new alliance though as they’ll have to make up your extra damage, but there are a lot of good alliances out there that are willing to do it for the right member (they’ll be carrying you for a bit, appreciate this as it’s boosting your ascension income).
While I’d prefer to suggest a Titan difficulty after I know your whole situation (your heroes, item usage, how many hits per Titan) I can let you know my experience with Titans. I’ll list what it takes to be a top 5 hitter, if you can be a top 5, you can afford to move up Titans.
On 4* & 5* You can be the top 5 with lvl 60 4* or specialized 3* Titan teams.
On 6* & 7* You can be top 5 of the Titan boards if you have mix of 60 or maxed 4* Titan team with some of the following:
Wukong, an armor debuffer, and Boldtusk/kiril +supporting strong colors.
On 8*/9* you can be top 5 if you use minor items (Mana, arrows, axes, health pots, etc) and if you have the majority of damage stackers in each color, strong supporting colors, and good strategy and know-how.
On 10* to be top 5, you will need just about all of the 4* damage stackers with options of maxed 5*, elemental debuffers, bigger items, and solid strategy and know-how.
I’m writing this on the fly so it’s up for debate, it really depends on a lot of factors so this can’t be taken as 100% correct. Some heroes can be substituted for items and vice versa. Shameless plug- If you’ve outgrown your 6-7* alliance, I’ve got a spot open for a quality member in Syndicate of War hitting 9* Titans. We’ll be in top 100 soon. We can talk further if you’re interested.
I hope this helps!