Legion is my favorite ship I usually always keep 1-2 in my end game late fleet... I know they aren't the BEST but they are really fun to use.
When built right I find them effective against Ordos and pretty much anything. I do prefer the XIV but I will use the base version with good success if I can't find the 14th ones, I typically build it as a hold the line tank, and will let the AI handle it for most of the battle while I pilot something faster (usually Oddesey).
Main mission goal is to overwhelm shields on capitals or slower cruisers as fast as possible from close range while tanking fire with armor and shield. The two large mounts just don't put out enough DPS to come close to taking down anything larger than a destroyer, and it doesn't have the speed to stay in range anyway, so I build it with unstable injector, high flux pool and flux efficient small ballistics like needler and railgun and machine guns and Assault chainguns and Flak Cannon or two so the AI can't overflux it. Fighter complement 1x Xyphos for pd while venting and 3x Dagger maybe 1 Trident usually. I give it integrated targeting unit which boosts ranges 40% which is way more effective than ballistic rangefinder ( @Alex why is rangefinder so bad? ) as well as Hardened Shields for 15% bonus to shield efficiency (HS gets slapped on all most of my combat ships)
When it's finally lumbered into a good central position I will switch to it and punish overextended enemy targets, burn drive right up into there face and let loose on all guns with Tridents on regroup. Then after their shield pops (which isn't long with 10+ weapons in range) let the tridents go engage to finish them off. Then I'll usually take command of another ship while the AI (aggressive usually) retreats to dissipate flux. Against heavier targets the Assault Chainguns are murderous. I've also found the tridents seem launch their astropos at very close by ships even when set to regroup floating behind the big lump, and they reload very fast this way and don't get shot down as much.
After burning in and popping the target, often the AI fleet will get mad AF and over extend and elongate chasing the offensive Legion, while I fall back pop shield on and off armor tanking.. This gives my fast cruisers and destroyers a perfect opportunity to flank them. Dooms plasma mines will make them regret this REALLY fast. Positioning is very important tho as it's Legion is so slow, if 2-3 capitals manage to surround it with one behind it's shield arc, it's pretty much done for and the braindead ships AI will manage to do that every so often. Aim these Naruto charges towards weaker sections of the enemy line.
Since It really doesn't have the speed to pursue anything or keep itself in range of anything, I find it performs best when used in this Anchor / Anvil role. The XIV missile barrages are icing on the cake but I've found it effective without them as well, as you can put the OP points into flux pool and better weapons, ITU, solar shielding, and/or heavy armor. Ordinance Expertise is REALLY good on Officer/Players when using Legion this way.