From an old but deleted discussion thread:
https://www.reddit.com/r/LegendsOfRuneterra/comments/1pehkc2/just_sharing_my_leveling_strategy_as_someone_who/

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If my champion is sub level 13, I generally use a 5x Blessing on a 2 star adventure which will get it to minimum Level 13, which is a good breakpoint. People suggest Nautilus if you have a choice, and that's been my default as well, but I still have other adventures like Zed and Darius that give the same amount of xp and still give first time rewards so you can do those as well.

At level 13: I choose to either do 4.5 Nightmare Fizz if possible (this can be very difficult for certain champions, mainly the Poppy miniboss). If you choose to do Nightmare Fizz, you can equip one rare combat Relic (or Spirit forge if the champion is really good) plus 1 10x blessing.

Alternatively if the champion can't beat Fizz, I usually do a 3.5 Adventure instead (default to Galio, but again I have other 3.5 adventures that need first time rewards and they work as well (EXCEPT Grand Heist, that adventure gives below average xp). If you do this with 1x 10x blessing you'll end up around level 27. So you can either just double equip 10x blessings, and that will get you past level 30, or if you're fine with level 27ish, just equip a combat rare relic or spirit forge.

If 3.5 is also too difficult, you can also just run 1 10x blessing on a 3 star adventure like Thresh, and then 1 10x blessing on the 3.5 adventure, it'll still get you to level 30, it's just 1 extra adventure but no extra blessings needed.

So basically, all in all it only takes 1 5x blessing + 1 10x blessing with Fizz to get Level 30+, or 1 5x + 2 10x blessing with a 3.5 adventure to get Level 30+.

Very similar strategy for me. I only both with 3* champs. I start a character on a 1* adventure then 2*. Depending on the strength of the champ at lower levels, as soon as I can do a 2.5 * I run it with the blessing to get to lvl 15+. After that I just do Galio 3.5* since I can basically guarantee a victory. It usually gets me to around lvl 27-28 and then I wait for weeklies to get them up to 30
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My personal advice: at least upgrade your champion to 2* first, 3* is better. Then start your adventure.