It becomes more even if one combine matching infantry units to each attacking artillery. But still any force of defending infantry units only, will dominate any force of heavy artillery (attack on 3) with matching infantry. See the statistics below:
7 Inf
Cost: 21 IPCs
Def: 7*2 = 14
No Units: 7
3 Inf + 3 Art
Cost: 21 IPCs
Att: 33 + 32 = 15
No Units: 6
1st Combat Cycle
Art combo scores: 15/6 = 2,5 hits -> 4,5 inf left
Inf scores: 14/6 = 2,33 hits -> 3 art and 0,67 inf left
2nd Combat Cycle
Art combo scores: 33/6 + 0,672/6 = 1,72 hits -> 2,78 inf left
Inf scores: 4,5*2/6 = 1,5 hits -> 2,17 art left
3rd Combat Cycle
Art combo scores: 2,173/6 = 1,08 hits -> 1,69 inf left
Inf scores: 2,782/6 = 0,93 hits -> 1,24 art left
4th Combat Cycle
Art combo scores: 1,243/6 = 0,62 hits -> 1,07 inf left
Inf scores: 1,692/6 = 0,56 hits -> 0,68 art left
5th Combat Cycle
Art combo scores: 0,683/6 = 0,34 hits -> 0,74 inf left
Inf scores: 1,072/6 = 0,36 hits -> 0,32 art left
6th Combat Cycle
Art combo scores: 0,323/6 = 0,16 hits -> 0,58 inf left
Inf scores: 0,742/6 = 0,25 hits -> 0,07 art left
7th Combat Cycle
Art combo scores: 0,073/6 = 0,04 hits -> 0,54 inf left
Inf scores: 0,582/6 = 0,19 hits -> no art left