With the Monster Lab, you can Rank Up your Monster with another 5 Ranks ★. Each Monster's Rarity can expand his level limit as follows

COMMON
45 ★
50 ★★
55 ★★★
60 ★★★★
70 ★★★★★

UNCOMMON
65 ★
70 ★★
75 ★★★
8 0 ★★★★
90 ★★★★★

RARE
80 ★
85 ★★
90 ★★★
95 ★★★★
105 ★★★★★

EPIC
95 ★
100 ★★
105 ★★★
110 ★★★★
120 ★★★★★

LEGENDARY
105 ★
110 ★★
115 ★★★
120 ★★★★
130 ★★★★★

MYTHIC
110 ★
120 ★★
130 ★★★
140 ★★★★
150 ★★★★